Since January 2011 I used WP7 for some months. I loved the smoothness but missed some features/software and there still where some driver related bugs.
Then I used a Android NAND MIUI rom for some months. Fairly smooth experience, some nice Android modifications, but always seemed to have bugs.
After that I used a Android NAND Sense 3.5 (sbryan12144's RCMix3D Runny) rom for 2 months. Many features, but not always a smooth experience. I liked it because I experienced no bugs.
I hesitated a lot (because of my bugfree experience), but since tuesday I'm back to WP7.5 and enjoying the smooth experience again + some extra features + fixed touchscreen driver + fixed camera. Only missing some good GPS software.