This is my nº4 phone from sony ericsson, i start at the t610/t630/w500 and now the w800i. The aspect fisic is almost the same, but much improvement has been made since the first t610, first and very usefull is the slot memory expansion, now i can put mp3/games/fotos/movies and even carry data with that, well placed (the slot), because you dont have to take off battery or someting to get access to the card.
The sofware is similar to other sonys with the major diference of the walkman player, the battery has always on the sony ericsson have a good live time, you can even choice when powering the phone, to chose if you whant to full power as a celphone or just the walkman funcion, the sound is also much better, it could have more power but is good quality. The backlight (flashlight)as enougf power to iluminate a room, very usefull when you are camping or other situations.
The design is prety much the same for the t610/t630, a bit larger, but the screen size the same, the keyboard the main change is a hotkey for walkman player. Im a big screen lover so i would prefer a bigger screen, and you can have some problem on the outdoors suny days, the screen is almost impossible to see anything if the sun is up the screen.
the main bug on this phone software is the camera shot, yes, its a bit anoying when you whant to take a picture and it take about 5 second to shoot, this probaly is going to be fixed on next firmware upgrade but until now its the main anouying thing i found on this serie.
Just living some of the tecnical info as request :)

Features of the K800i include:
2 megapixel camera with autofocus, 4x digital zoom & photo light
Video camera & sound recorder
Display: 262k colours, 176 x 220 pixels
Music player
FM radio
40-voice polyphonic ringtones
Polyphonic melody composer (MusicDJ™)
Speakerphone, voice controls, voice mail
Messaging: SMS (with predictive text), EMS, MMS, Instant messaging, email
Java games (embedded & downloadable)
Animated wallpaper
Picture editor and effects
Personal organiser functions
Picture phonebook
QuickShare™
Connectivity: Bluetooth, USB, Infra-red
WAP 2.0, GPRS
Memory: 34 Mbytes plus 512 Mb Memory Stick Duo™
Triband
Vibration alert
Size: 100 x 46 x 21 mm
Talktime: 9 hours
Battery standby: 400 hours