Epic feedback time.
This is a fantastic co-op game, and a "good" single player game.
TLDR: If you don't like this game on some level, you have no soul. 
For me it's the kind of game I can easily put 50 hours into single player and have a lot of fun. MP accelerates how fast you get through the game, but it makes up for it by being more fun with company. (And throwing a ton more enemies at you.)
Terraria does several things very well:
-The World. It's fucking huge. Even though it's "randomly" generated, it's fairly procedural too. It's got a structure that's fun to explore and does some cool things purely on it's own at creation. And it's all modifiable, every last inch of it, for game play reasons or just for cosmetic ones. It's a true playground that is persistent, goes from SP to MP with ease, and is separate from your character. Characters and all their personal inventory can bounce between worlds, between SP and MP, whenever they want. You can make worlds just to farm their riches and resources, then have worlds just for PVP or to make pretty or hellish, and do it all with the same character if you wish.
-Exploration. The platforming combined with the ability to change your environment, combined with the excellent light mechanics, combined with the randomly generated biomes and distributed treasures, makes for a very fun exploration experience. You dig, find ore, notice hollow spots in the earth, cloaked in darkness. You dig through to them, miss a jump, plunge a thousand feet into pure darkness and drown in an underground lake while being assaulted by slimes and skeletons. And when you're lucky enough to stumble across the really good stuff.....
-Progression. You really feel the progression in this game, and getting better stuff is candy not unlike the best MMO you've played. You start out very humble, with no real tools or power, meager health and a measly 6 block-high jump. The magic items you collect and the tools and weapons you make drastically change the way you play. You start out needing to build platforms out of wood or stone and dirt to get pretty much anywhere. The first 500 feet of your mine at least will be covered in torches to push back the darkness and platforms to make it possible to go somewhere quickly and safely. Later, on, you'll have tools like grappling hooks to repel through caves and up cliffs, magic items that will generate light without you having to manage them, better and faster equipment that mines special resources and mines normal resources very quickly, weapons that double as tools....
And then there's the accessories, the things you have to find instead of make. Accessories that makes you run faster, jump higher, double jump or safe fall. Stuff that lets you teleport. Stuff that makes your life regen over time. Stuff that lets you navigate water filled areas easier.
The progression and the way it alters how you play the game is perhaps the best part next to the generated worlds and exploration. There is already a ton of content there. The only bad part is that it has an end!
-Variety and amount of features. Tons of weapons and items and cosmetic stuff. The different biomes with different enemies, visual effects, special rules, special items and things like the Shadow Orbs, which when broken by your hammer either summon bosses or cause meteorites to strike the world (which you go find and mine the ore from.) The devs aren't really concerned about balance so much as they are making sure everything is fun to use. Some stuff has to be found, some found stuff can be upgraded, or stuff is built by the player from the probably 120+ material components. It's the little touches too. Cobwebs, a seemingly innocuous cosmetic thing, does a ton of stuff. It slows your momentum by like 95% when you touch them, often saving your ass while caving. You can farm them to make silk for making beds (which let you reset spawn points), for making a bad ass late game armor or just for putting up around places to make them look creepy cool. That kind of attention to detail is everywhere in this game. My personal favorite is the underground grass that only grows on mud. The grass causes high powered healing mushrooms to grow on it. If you add mud to an existing patch, it spreads and you get more mushrooms. You can make a whole mushroom farm that supplies your healing potion industry. And it even glows faintly in the dark.
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Things I don't like:
-They had to replace some sprites, mostly armor. The reworks are not inspiring to me at all. Hoping they get more work or a better spriter.
-The game isn't as fun to me in MP if people don't stick together. Also having someone come into your world with advanced gear throws off progression a lot, and to me that's really important. Going straight to the end game of Terraria is missing 1/2 of the experience. This is a game to take slowly, meticulously and with your friends close by instead of going off to do your own thing. Once you get the best in Terraria, that's it. You've reached the top of the mountain, there's no other place to go and very few mysteries left. And it's the mysteries and the struggles to discover them that are what define this game. There might be some room for a sort of open world server, but it's definitely not my bag, because players will scour it like locusts and once the treasure is gone, it's pretty much gone other than rare drops. This is not a game I would play with the general public on a regular basis, it's waaaayyyy too gear focused. Think of playing it with total strangers like playing Diablo II if the best loot was only found in one time chests. When you're building a house so you don't die at night and someone else is raiding chests and goodies as fast as they can find them, it can start to be annoying. Play SP to find loot. Play MP so you have someone at your side. Sharing and multiple players also tends to spread the loot out more, which slows down your progression, which makes everything in the game relevant for longer.
-Taking the game slowly is very time intensive. But it's totally worth it in my opinion. If you're into building stuff and taking your time, you can spend a solid three days and still not completely tap the game out. (I know I have)
-PvP is utterly grief-tacular. You don't have much to lose, but the unrestricted nature of the game mechanics means someone can destroy something you spent 2 hours creating in a matter of seconds, and then effectively "salt the earth" once they're done.
-The spammy nature of fighting and of weapons and such doesn't paint a very deep picture of PvP to me. It may be fun, but spammy, oh lord is it spammy. Bosses will regularly kill you, then come to your spawn point and kill you again, and you just flail and do as much damage as you can each time before you die. Large boss fights are just a train wreck and the screen explodes with all sorts of stuff. It's just very chaotic, and the game leans heavily on the side of preparation: gear, drinking potions at the right time and terraforming if necessary, and less on the side of skill. In the dev's opinion "Difficulty in Terraria is about where you go with what you have." Like I said, they don't sweat balance too much.
-I had to get an autoclicker for this game. Holding down the mouse to mine while moving it 8 pixels at a time to maximize your digging time, hundreds of clicks to craft large stacks of items, furiously clicking to swing weapons or fire missiles, this game will wreck your hand, your wrist and your forearm after 5+ hours of playing. In small doses, it's a non-issue. But after many hours you start to feel it. It takes probably 8 to 10 minutes of straight digging to get to the bottom of the map.
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Is it worth the money? More than any game I bought this year, easily. It depends on your gaming background, but Terraria hits a very, very sweet nostalgia spot for many people. It draws on the best of my childhood: Mario, Castlevania, Zelda, Metroid, Final Fantasy, and fuses it to stuff that's awesome today, namely digging and block building based games. There's lot of promises for future support from Re-Logic: more items, enemies, biomes, modes, mechanics, the works. These guys are already making bank on Terraria, they've got the freedom to do anything they want.
I won't lie, the game definitely hits its peak and then there's nothing left to do but goof around and build stuff. But they were aiming for a release initially months from now, and people demanded it release asap. I've already put 50 hours into Terraria with friends and some SP, and haven't maxed out because I don't want to rush. But it has been a total blast getting here, and if they really pump more content into the game, it could be good for hundreds more hours. Like people play a FPS obsessively after they've done their dues, Terraria is shaping up to an RPGer's FPS equivalent.
