I've had my Wing for about 8 months now. (Went from a Treo 650 --> 680 --> Wing) As others have said - the OS runs slow on this, due to a slow processor. (Screen lag is evident switching from horiz-->vert or vice versa, getting programs to load, even accessing the start menu) The lag doesn't bother me that much though - I'm used to it by now, and I don't even notice it anymore. Some people might be bothered by it though.
One other thing to note - not sure if it was fixed in future revisions of the phone, but the buttons on the front of the phone tend to peel/flake off. (The Windows, OK, menu keys on the front of the phone all have faded paint) My friend's Wing also is doing the same thing, and appears to be common with many people. (Granted, I don't holster mine and just carry it in my pocket)
Another thing that irks me - it seems a little buggy. Lately, on weekdays my alarm has been going off at 6:15, and I don't have one set for that time. ???? Also - the sound has now defaulted to some weird "beep" when pressing the end button among other various things that makes sounds. I fixed it by doing a hard reset on the phone and deleting all the data, but it came back about 2 weeks later.
Lastly - the sliding keyboard - while cool, and it's nice having large buttons/keys - it lately has just become burdensome...having to slide it open everytime to reply to a message, type anything, slide back. If you're firing off rapid fire texts, replies, etc, it's just annoying. (If you have it on vibrate and just leave it in the slid out mode, it's pretty noisy)
All in all a good phone - I've loved it, but I'm going back to a Treo. (Probably a 750 though, I dig the Windows Mobile OS now) Just make sure the things I said above are fine with you.