Well i have worked with Win mobile 2003,5,6,6.1 and now 6.5 on my HTC TP2,well the difference between 6.1 and 6.5 isn't very much but 6.5 is better customized and more user friendly IMO and a little faster too.
About HD2,i disagree because i am waiting for more reviews,and i think except its size its a great device,the first capacitive win mobile + a great WVGA screen and powered by a fast Snapdragon CPU,so i think its a good device.