I had to jump in and vouch for High Tech here. I have personally owned 3 HTC going back when it had ties with Dopod and I am currently using the HTC Touch Color. Beside a few "standard" wm6 glitches, I have found the phone to be the best I have used in PDA phones so far. Sure, it's not the fastest thing you can find and it doesn't have the fancy interface of OSX or maybe symbian, but it's a very solid device and I am a happy 8 months owner. As with any WM devices, you really have to take the time and patience on using and configuring the phone to your liking, once that's completed, the phones tend to grow on you. I am actually debating the 3G iPhone and the Diamond as well, but I am withholding judgements until I get to play with both. Hope your dad likes the Diamond and post some more feedbacks!!
--Jack