Compares nicely with the W580i
I've had a W580i since it first came out (and it's certainly the best phone I've ever had - and I've gone through quite a few), and they sound like very comparable devices. The W910i looks a bit slimmer, and it has an additional camera, and HSDPA instead of EDGE, but otherwise they seem pretty much identical.
The only downside to the W580i is the same one mentioned regarding the W910i - the charge/headphone connector going on the side. I ended up getting some Bluetooth headphones to mitigate the nuisance of the headphone adaptor, and now I have only one device in my pocket where I used to have three (phone, iPod, and PDA); of course, my phone+PDA combination has been taken care of nicely by Sony Ericsson phones for quite some time.
Also, the shake-to-shuffle doesn't seem to actually do anything, but I'm actually a bit glad for that since I like listening to music with album-based playlists (generated by SyncTunes for OSX) and I'd hate for my music to get disrupted every time I have to run to cross the street or whatever.