I've had the following setup for the over 6+ months:
k750 + 2gb Pro Duo + Shure E4C + Volume Control (comes with E4C)
The K750 is the same phone as the W800, just has a different firmware. In the UK:
- K750 looks nicer (black)
- W800 has a better joystick, comes with 512 pro duo card and 3.5mm audio adapter, and better earphones (it's still crap).
Both are good phones with solid reception and lots of features (loudspeaker, able to play games and listen to music simultaneously, etc).
As a music phone, it has pros and cons:
+ battery life is good (excluding phone calls, it easily outlasts my ipod nano).
+ has equalizer (the presets aren't too bad)
+ convenience of carrying one thing instead of two.
+ phone is compact in comparison to other music phones.
+ i find it charges much better than my nano.
- i find the volume is too loud even at the lowest setting (hence the volume control)
- there's noticable hiss at low volumes (rectified by jacking up phone volume and counterbalancing with volume control).
- sometimes (not often) get "Failed" error message when it fails to read the memory card (which usually can only be rectified by rebooting the phone).
- user interface for music / playlists is pretty crap (i think the W800 is better than the K750 in this respect, but both come up short vs an ipod...)
- need adapter to use 3.5mm jack (you can get cheap ones off ebay which don't last long, or u can get the official Sony Ericsson adapter which costs more but is much more durable).
I've seen and played around with my friends N91, but i find it frankly way too big and heavy (it's as large as some smartphones). Despite all it's weaknesses, I think the K750 is a pretty good music phone.