I can reply to this from personal experience. My phone, until Sunday, was an EVO. It was a wonderful piece of hardware with great software. The connection between man and machine was natural and strong. It was my plan to have this for a long time. Then Sprint raised their prices, even on customers with contracts and I switched to T-mobile after more than 10 years on my old carrier.
Luckily, a local seller had an HD2 with Android loaded. This gave me the chance to try it in person, instead of risking a working phone or buying through the mail.
In general it works very well. Other than the few small glitches which will be noted in the list of differences below, the performance is absolutely as good as EVO. Everything moves quickly and properly on screen; the home key always takes you instantly home; it doesn't get hung and pause; etc.
This is the list of comparison items, regardless of whether they are glitches or advantages for the HD2, or merely differences between the two.
1. Difference - While the phone is the same size, the form factor is slightly different. The HD2 is a bit thinner and thus easier to hold, but only the EVO has that spiffy little "kickstand".
2. Advantage - The HD2 has "real" buttons below the screen, rather than touchscreen type buttons.
3. Difference - There are five buttons, instead of four in the row on the bottom. It has a home button and a back button, which work just like the EVO. The "Windows" acts as a "menu" key, with the search function on the menu in some apps, like it would a phone without a search button. I always thought the dedicated search button was rather useless, so I don't miss it. There is also a "hang up" button, which is nice because you can end a call without looking at the screen. You can still use the "End Call" red virtual on-screen button as well.
4. Advantage - There is no top button for on/off and sleep/wake. The "hang up" button makes the phone sleep anytime you are not on a call, and either the talk or "hang up" will wake it. I find this easier than the EVO.
5. Glitch - The "Stocks" app works for the major indexes that are pre-programmed, but goes to a "force-close" if you try to add a stock. It does not make the phone crash but just closes the app. I am hoping there is a work-around, but I may just have to find a different finance app.
6. Glitch - The Speakerphone icon needs to be turned on, off and on again during a call in order to make it work. Once that has been done it can then be toggled normally. This is minor, because I am just used to "triple-clicking" on the speaker the first time to make it work.
7. Glitch - Sometimes audio on incoming calls has a somewhat "digital" sound to it. This never prevents easily understood conversation and probably is still as good a many phones ever sound, but it lacks the richness of audio that one might otherwise get from this great phone or the EVO. The other person always hears you clearly.
8. Advantage - The "messaging" icon brings you to messaging every time on the HD2. On the EVO it usually went to a blank screen, from which the "back" key was needed to see any actual texts.
9. The "Visual Voicemail" on the EVO is specific to the Sprint network, so you would use regular voicemail procedures with T-mobile. They are supposed to be developing an app for this feature on their network. If they do it for android, I would expect that it would work on the HD2 with Froyo.
10. Minor Glitch - The signal strength bars sometimes all show flat lines when the phone wakes. Usually they go to a normal indication shortly after it starts being used, even if no call is made. This does not affect call quality, nor does it stop you from making calls, receiving calls or using text messaging.
11. Similarity - Battery life seems good with normal use and the auto-updating/synching off. Just like the EVO, it is more limited when these "features" are running, or if one does a lot of browsing.
12. Networks - T-mobile's HDSPA seems more developed in available almost everywhere in town, where as Sprints 4G only works sporadically in certain places. HDSPA is faster than Sprint 3G, but not quite as fast as 4G. Latency is low, with similar ping times to 4G, but upload speeds are lower than 3G. Overall, it feels like better, more consistent internet service that works faster on in more places for standard browsing purposes.
13. Advantage - In the HD2, the FM radio works with standard stereo headphones. On the EVO, it works on cellular headsets, but the average stereo 3.5mm plug shorts out its antenna.
14. Difference - The EVO has an 8mp camera, but the HD2's 5mp seems to take better pictures. The EVO also has a front-facing low-res camera for video calling.
I have not tried the Market, nor social networking apps, so my comments do not include them. SlideMe, the competing app market works fine. Maps GPS, mp3 playback, and YouTube seem fine as well.
It should work the same on any GSM carrier (except item 9), if the phone is unlocked, or locked to the carrier you are using.
Builds are coming out frequently, so I would expect that the glitches will end up resolved. Overall, it has been a good change and I am pleased with my choice of T-mobile Android phone.
I hope this helps...
Sam
P.S.: Android on HD2's is becoming common enough that the folks who run this forum may want to consider listing it with the phones, and treating it is an Android-capable phone for forum purposes. HTC's recent announcement that they are not planning to offer Windows Mobile 7 for the HD2 will only add to the pressure to make it Android.