Try the demo.
 
Personally I found it perfectly adequate to play alone.

It's certainly harder alone, but it's less annoying because A. you can take your time and B. you don't have to deal with all of the different levelled up c haracters on different people's profiles. 

I played it alone and it was harder than I expected. I actually had to grind levels to level up, but it isn't hard once you do that.

I played through the game in single-player and found it a blast.  Not much grinding was required since the pace of leveling up is fairly consistent across the full game and any experience earned on a stage where you lose all of your lives is kept, but it does take effort to earn enough cash to afford the more expensive items in the store.

I liked it a lot and played alone, so yeah. Buy it if you're a fan of old beat em ups and Scott Pilgrim.

Lucas Lee trashed me a couple of times at the beginning.  
 
I went back to the first stage and plowed through it a few times (focus on purchasing CDs from the Music Store) and came back to show him a thing or two about skateboarding... (TO HIS FACE!)

it's fun, but god damn is it hard.

It is really hard by yourself. Other players can revive you in co-op.

It's....ok solo. About the best you can say for any beat-em up. They're designed to be played with friends so it's always better. Have you play Castle Crashers alone? It's similar.

Beat the game last night alone and I had a great time.  I also leveled Scott up to the max level and maxed out his stats.  I'm playing through again tonight or tomorrow to beat the game on the hardest mode.  Then I plan on beating the game with all the other characters.  Sure I might get a friend to play along sometime this weekend but I'm having a great time playing this alone also.

It's fine in single player, I finished it with the five characters all by myself. It's a lot more fun with a few upgardes though. Some grinding may be required