The characters are a little too small for the vita screen.
If you go through the campaign mode you'll have a little backstory for each character and a sequence of game modes that you'll have to run but it's quite easy to get through. The final boss is quite uninspired and it's just there to create the scene of story and progression, but it's just there as a punching bag after you fight the other characters that you've been fighting all along. There's an interesting balance of characters where everybody pretty much feels that they have a good shot on fighting the other parties in an equal basis. The online portion is quite annoying to me since there are people that locked down the routine and it's quite a waste of time to spend on those fights.
The big problem for me is that the characters will look pretty small and it's hard to see what's going on most of the time. It's not unplayable but not too appealing to sink too much time due to the real estate screen that you get.
The trophy collection is quite easy to go through, I was able to get a platinum with fairly minor sweat. It's a good diversion but I can see this game being a very nitchy community of people to wish to have a Smash Bros. on their Sony system(s). If you can get the cross buy I'll suggest you to go with it and take advantage of the PS Vita.