I just watched a couple videos about LauncherPro and Helix. It seems to me that those UI were developed to imitate, in alot of ways, the sense UI. So, I guess in my mind, it would make the most sense to stick with the UI that it seems everyone is imitating. I like the sense UI and it seems to work very smooth on my phone. I'm one of those people that lives by the theory "if it isn't broken, don't fix it". I got my phone two weeks ago, did the OTA update, and everything is smooth. Battery life is slightly above avg (from what im reading on the forum), and nothing seems to be misplaced (knocks on wood).
Maybe its just cause I'm not so worried about the speed difference, but I don't see the reason behind messing with your phone to pretty much get the same thing as you already have. People want to get rid of the sense UI to get LP or Helix, and then they download beautiful widgets or some other app to replace the standard HTC app. Why go through all that trouble? You already have the apps, and a UI that looks like helix and LP, so why not just keep it? JMO .... Plus, is the speed really that noticeable/important?? (legit question, not trying to sound like a SA)