I'm almost done with the hero campaign. This is one freaking long game. The interface takes a while to get used to, but I've eventually grown to like it. I don't know how different it is on the PSP, but the stylus works well for me.
The really good thing is that there is a huge collection of cards available in the game, so I've been able to completely change my deck after almost every chapter, and fine tune it while I'm playing through the individual missions. I've only been able to really focus on decks with X-Men and Avengers (the early ones were a mash-up of different teams), so there's a lot more options to explore. I got this when it first came out, and I've only briefly played other games (except for Godfather on the Wii).
I'm probably more addicted to this than I was to Puzzle Quest. It's just that awesome to me. And for anyone having trouble learning the basic rules, I think the UDE VS system homepage had an interactive flash tutorial. I remember trying that out a long time ago, and it was pretty good.