I'm using an iPhone 4 on T-mobile now.
1. Had to cut down my mini-sim to micro-sim size. That was not difficult.
2. Had to get it from Europe (UK) so it was factory unlocked. Not difficult, but not subsidized.
3. Works nicely except only on EDGE, not 3G - presumably due to the frequency issue. In theory, it should be relatively easy to open up the new frequency (e.g. no additional antenna/chip needed etc, just tuning to another frequency like a regular radio). Hopefully the chips support it. IIRC they can.
4. No visual voicemail. Not a big deal.
5. T-Mobile on iPhone 3GS (on 3.x and 4.0.0) was having trouble receiving MMS, but sending okay (e.g. Google "the media content was not included due to a picture resolution or message size restriction"). Not sure if iPhone 4 running 4.01 is still doing it.
6. Since EDGE and not 3G, can't using internet/mail etc while on the phone. Not a big deal.
7. Gets equivalent or better reception in the areas (Florida only so far) I use it with iPhone 3GS's on AT&T - e.g. same areas, bars etc. (EDGE vs 3G of course).
8. iPhone 4 has performed equal or better in reception and dropped calls in the 2.5 weeks I've been running it on T-Mobile US as compared to previously with iPhone 3GS.
All in all, it (and 3GS and 3G) have worked great on T-Mobile USA with the exception of 3G vs EDGE, so it would be welcome to have it officially supported.
BUT, I'll believe it when I see it. Seems unlikely that it would launch in 2 months, 1 week. Could be just a software upgrade to tune correctly, but seems kind of rushed given past history.