i reckon that as u already have a k800i then the k850i will be no major changed but indeed, an upgrade. i got one yesterday, first sony phone i've ever had and i am impressed with it. screen is very nice, speaker is loud, camera is good and theres all sorts of features ure already used to on the k800i that'll be there. from what i've seen of it so far i'll give it a 7 out of 10, i'm not used to the txting as i've had an xda orbit for last yr so thats not really a fault on the phone, just me needing to get used to it. the touch buttons are nice if u get them in the right place, u may find u need to push it a couple of times sometimes. the only complaint i have, and its a small one is that as the phone is quite square it hurts my ear a bit when i hold the phone to it when speaking and the audio quality of the call isnt as good as wot i've had on previous phones.
all in all a nice phone, i would recommend.