On Friday I managed to procure a Sony Ericsson K750i and after spending the weekend with it I'm very impressed. In basically the same form factor as the T610, they've jammed in a 2 Megapixel camera, MP3 player, games and all the organizational tools I use in my Treo. While it's not a Treo replacement, it's definitely a great alternate phone for one who has come to expect massive functionality. I was able to sync easily with my Mac (after following this hack) to get all the calendar and address book data I needed. The high resolution screen afford easy viewing of both calendar and email.
The user interface on the 750i follows the same paradigm as other Sony Ericsson handsets. It has been improved, however, to take full advantage of the phone's more powerful processor. Little things like transparency and animation make moving around the device more pleasurable and well organized menus help you find what you're looking for pretty quickly. I especially appreciate how the interface rotates when the phone is in its horizontal camera mode.
For a fully detailed review, check out Mobile Review.