I love the trackball on every bb i have tried. So much better than the side scroller.
I just went to the T-Mo store and played with the Flip for about 5 minutes. I thought the trackball was great. Its set down a bit so its slightly different than the trackball on the pearl/curve/etc. However, they did a good job smoothing the keys surrounding the trackball, so it feels pretty good on your thumb.
I'm waiting on the storm, but if i was buying a new phone, and didnt previously have a pearl, i would probably give the flip a try. but, since I have a pearl, i don't think I would upgrade to the flip. rather, I would wait for the Bold, or the Storm