Well, I am not going back to WinMo 6.1 after using this. Initially the beta roms that came out were handful, but as the official release is getting closer, the roms are getting mature and are relatively bug free. Resource use is same. ( optimised roms actually have lower ram usage than HTC WM 6.1)
There are still some issues and small bugs with the Manila 2.5 ( If you restart the phone, wifi is turned on by default. You have to turn it off manually ), but its not a big deal.
The UI you see above is HTC's Sense UI that we saw with their android phones. Ported for Winmo.