I have only ever owned one HTC in my life, the HTC Touch Pro and I hated it! I have windows and I hated that phone so bad. One thing that slowed my phone was the UI over windows.
I'm very new to Android, only played with it a little at the stores. With the HTC EVO is their a UI iver android and if so does it get in the way of the android experence or does it slow the phone down?
I'll give you some advice since my last phone was a Touch Pro. I actually liked my Touch Pro initially, but the longer I had it the more I realized how time consuming and frustrating Windows Mobile was. I also had a slight hesitation about getting the EVO due to the lack of the physical QWERTY keyboard.
Let me tell you that Android by FAR exceeds the capabilities of Windows Mobile (w/ exception to bluetooth voice dialing, which was the ONLY thing I wish was just as good as Windows Mobile). But I can say that I no longer feel like I'll ever need a qwerty keyboard ever again.
Getting to your direct question regarding HTC's Sense UI. It really is an updated version of TouchFlo 3D, but it FAR exceeds how TouchFlo 3D worked and I think a big part of that is due to the Android OS. I actually like Sense UI a lot. TouchFlo 3D, on the Touch Pro, was absolutely HORRIBLE. The only reason I didn't complain about it was because it was first available in the US on the Touch Pro, so all the kinks of the OS hadn't been worked out. I think it probably helped HTC build Sense UI to better suit the consumer's needs. The one thing that HTC still hasn't figured out is how to get their UI (Sense in this case) to go into landscape mode. Just like TouchFlo 3D, there's no landscape mode on Sense UI. Majority of the apps automatically rotate to landscape, but if you have your phone on the desk (w/ the kickstand out) or in the car (on a mount) in landscape, you'll need to turn your head sideways initially, just to get to the app or apps that you want. They've kept some functions from TouchFlo 3D on Sense UI, but it really is a much improved interface from TouchFlo 3D. To a certain extent, you'll have a good advantage from having the Touch Pro w/ TouchFlo 3D because you'll have a little 'bit of an understanding of how the UI works. The great part is, because of Android, the customization doesn't take up anywhere near as much time as it did on Windows mobile! The things you'll recognize from TouchFlo 3D that are available through Sense UI are 1) the scrollable emails or messages in one page (in the form of a Widget). Sense UI allows you to choose different widgets that'll best suit you, 2) Scrollable contacts, 3) Bottom scrolling tabs are now ONLY when you've opened an app (and you really don't need them much, unless you're trying to filter what you're looking for), rather than how it is in TouchFlo 3D as the main part of scrolling through the different screens in the UI, 4) Scrollable pictures. I'm sure there are a TON more things that you'd probably recognize as something that was derived from TouchFlo 3D, but I'm telling you without a doubt, after the first day (launch day) that I had my EVO, I never wanted to go back to Windows Mobile EVER again (or anything else that Microsoft comes out with for mobile phones). TouchFlo 3D could only be customized after searching through the internet to find different Windows Mobile programs (cabs) that would allow for you to change the UI look. On Sense UI, you have 7 windows to work with that allows you to put pretty much ANYTHING you want on each page. AND that's standard on the phone. You don't have to go searching, and searching, and searching to find out how to make changes in the system, JUST to get the phone exactly how you'll need it. It'll be ready to customize out of the box!
That said, the GREAT thing about Android is that they have replacement UI's available (LauncherPro, ADW, and others) that are available in the App Market that can work in place of Sense UI. And those replacement UI's have the ability to work in landscape mode. The tradeoff is that you don't get the HTC widgets that come with the use of Sense UI. Of course, as I'm sure you'll find out, there are alternatives to those widgets as well. Even going further than that, you don't need to "root" the phone and load up a custom ROM to do ANY of this! In Windows Mobile, the equivalent of "rooting" is something that was standard (basically administrator access to change the registry) and much like Windows Mobile you can load "custom" ROM's onto the phone (assuming you "root" the phone). My take on rooting is that it's GREAT for those that really have the time to make sure they understand what they're doing and want to figure out what works for them. It's also great for those that don't want to pay extra for the mobile hotspot function or tethering. For me, personally, I don't really have much time for that anymore. Back when I had Windows mobile, I felt like it was worth the time because Windows Mobile was lacking as far as my needs as a user. With the EVO, Android, and Sense UI (vs. Touch Pro, WinMo, and TouchFlo 3D), it's not even a fair comparison. The EVO, Android and Sense UI blow the TouchPro, WinMo, and TouchFlo 3D out of the water no questions asked.
Hope that helps you make your decision.