I like the horizontal App browsing a lot like the Palm Pre. And it is responsive and smooth decent looking UI
The only problem I have with TW is that the default launcher apps, (contacts, Messaging) in the dock can't be changed..
Specifically I use handcent over the default messaging App, I prefer to be able to change those shortcuts. I also like to customize icons and themes.
I had no other problems with TW .
But the locked in launcher/Dock apps are why i choose LauncherPro over TW.
I highly recommend Launcher Pro over TW if you like to customize the dock apps, and like to tweak your icons.