So far as I can tell the +/-'s compared to the diamond:
+'s: Bigger screen, 3.5 headphone jack, slightly bigger battery, poss faster processor but cant remember what it is on the diamond. Slightly better camera
-'s: No d pad, HTC have forgotten to add a decent flash *AGAIN*
Sure it looks great, but Im happy with my diamond, and I really cant tell why I would want to trade it for this device. Im sure its very good and I wouldnt say no to a free one but then I'm not convinced there is enough specs to make me want it over the diamond.
Now if HTC finally got their act together and put a decent camera with a decent, and I mean decent omnia style flash together with the 5mp camera, had a decent frame rate for video recording, and added a few other bits like divx then it might twist it for me.
Still happy with my diamond, and struggling to like or dislike my omnia! I really dont need any more phones right now!!