I too came from using the Treo 700P. I agree that the MMS and SMS was better than that of the Omnia, as was Chattermail.
The keyboard will take some getting use to. I purchased the SPB keyboard and it makes typing easier for me.
I don't use the widgets. They aren't customizable and I didn't find them useful. I think the standard windows mobile interface is better.
I also purchased elecont launcher so I can launch my favorite apps from the today screen as I did with the 700P.
The contacts app on the Omnia isn't as good as the Treo either, but the Omnia has BT voice dialing so I don't really need to use it.
There is no lag, no odd reboots, Opera mobile is much better than the Treo's browser, the camera is much better, it can multi task, no skips with playing music, and video streaming is a lot better.
I've installed:
elecont launcher
elecont weather
vito ringtone editor
flash player 7 ( I can actually view some flash content)
youtubeplay
musicID
Google maps (doesn't work with the standard keyboard)
pocketTV (streams mpg video)
screen capture (same as snap for 700P)
vox (blogging software)
uno
poker
bill buddy (reminds me when to pay my biils)