Mmm
I've used U3, and while it works well, and is a very slick concept, I really dislike the fact that launches every time you insert and remove a memory stick. I'm also not terribly impressed with their selection of programs.

I do, on the other hand, like portableapps.com, portablefreeware.com, and tinyapps.org, mainly because they have (mostly) OSS and freeware apps. Not saying there aren't lots of good payware apps, but I do own a Linux-based tablet for a reason!