I have a htc touch cruise. And played around with a diamond before, it's significantly smaller, much better screen resolution. On the bad, It's battery life is not at all good even among HTC's. I prefer to have both 850/900 bands, and I do swap SD cards specifically for GPS when i need.
If you are familiar user of htc and other windows mobile devices, then I think I would recommend it. But like some of these other ppl have noted about complete touch screen control, you may find it very annoying. For one example, With no tactile feedback, constant/auto screen backlite shut off and screen changes away from the keypad, It's a major pain in the ass to dial to 1-800 type service line where you need to do touch tone commands.
The main justification of my use are viewing/editing Words/Excel document for me. Viewing scheduel in PDF. Occasional browser usage. And GPS. There are so much many other window mobile application too.