I was a bit hesitant buying this at first, seeing as I don't have any friends to play this with, and it's designed as co-op.
I wish I bought it earlier.
It is well possible to beat the game single player, though at times it can feel a little overwhelming. I've beaten the campaign, but have not beaten the DLC's yet, as I have yet to try them.
If the only thing preventing you from buying this is the fact that it's designed for co-op, just get it. If you get stuck, watch a guide. Fryedegg's channel on Youtube was a great help to me on some of the harder levels. I have yet to try the levels on Nightmare mode (Hardmode) seeing as I'm farming skulls to make things easier. If you're thrown off by me saying you need to spend time farming, you don't. I just am to make things easier.
You can reset the upgrades, refunding 100% of what was spent, which is a feature I wish i'd figured out before I was halfway through the game.
This game is extremely fun, moddable, has a lot of replayable value, beatable in single-player, has a co-op mode, is better when eating tacos (What game isn't?) and... well... yeah. Get it.