I'm a huge fan of the paper game, and it's a family favorite as my daughter likes it. There's a fun kinetic quality to moving the marbles.
The ipad adaptation is excellent and very well made. But it lacks any kind of campaign, which I think is a missed opportunity. For board games that are big and complex and take hours, or have deep strategy (e.g., Twilight Struggle), standalone games work. But for PE, you're in and out in 15-20 and how many times are you going to play that way? It's not like there are tons of different strategies to try out.
I think the game really needs a storyline, campaign, or some kind of pleasant treadmilling because even though I'm a fan, I played the ipad version a half dozen times and never went back to it.