Given that this is my thread, I feel it's about time I chimed in here.
I thought TW was fine at first, albeit ugly - the homescreen indicator at the top is GIGANTIC, many of Samsung's widgets take up entire screens for no apparent reason, and the iOS ripoff of a horizontal app drawer looks cartoony, unorganized and amateurish - at least iOS lets you rearrange apps. Plus, why did Samsung feel it was necessary to put randomly colored blocks behind each app? Did somebody seriously think that was an attractive feature? (Seriously, if you think this is attractive, please explain to me why).
My other problem with TW is speed. To everybody saying that TW is fast and stable - have you ever tried anything else? It's sort of like saying IE is great when you've never tried Firefox, but after trying FF, you can never go back. Compared to Launcherpro, TW is slow, choppy and unstable, with constant FCs and lag issues. LP in contrast is consistently buttery smooth, and seems much more stable. Admittedly, LP may be faster since I only have 3 screens compared to TW's 7, but this brings me to my next point, and the real killer for me: lack of customization.
I know that this won't matter to the casual user, but my phone is my mobile computer, and just like my computer, I want it to represent my personal style and taste. It's MY phone, not Samsung's, and I'll be damned if they tell me how my phone should look or operate.
Why can't I choose the number of homescreens I want, or decide where my default screen should be? Why am I forced to choose between horizontal scrolling or a list, instead of the vertically scrolling app drawer used by literally every other android phone in existence? Why can't I remove or hide certain apps (like Sprint crapware) from my app drawer?
And why, for the love of all that is holy, am I not allowed to change my dock apps? This is probably the single more egregious flaw present in the entire phone, and especially frustrating given that other flavors of TW don't suffer from the same restriction. Let me explain why this is a problem for me.
Early on, I adopted Handcent for its improved functionality over the default messaging app. Too bad, says Samsung, I'm instead forced to use the app THEY chose for me. Additionally, why did they feel it was necessary to include a dedicated contacts button when there's a contacts tab IN THE DIALER? I'd gladly sacrifice the extra button press in return for an app I actually WANT on my dock.
TL;DR: TW irks me for the same reason iOS irks me - it represents a fundamental arrogance on the part of the manufacturer, who is in essence telling me that they know how my phone should be used better than I do. I abandoned the iOS platform for Android for this very reason, and was disappointed to discover that Samsung went against the entire philosophy of Android by severely limiting the ways I can use my phone. I'm being doubly punished by having to wait for them to update 2.2 to this baked-in crap, even though removing it will be a first priority for me upon its release.
Samsung, you may be able to make a sweet piece of hardware, but you suck at UIs (Instinct, Behold, Behold II, TW, etc).
[/rant]