I know in my review of the K750i I said I wouldn't give that phone up. And indeed I still have the K750i used for that review.
The K800i expands little in the way of functionality. The only real significant addition being the capability of 3G video calling.
The build quality, while not feeling as heavy or solid as the K750i, is still what you would expect from a Sony Ericsson mobile phone. Though made slightly larger this has allowed for a bigger, clearer screen. This in turn allows for better picture taking and video capture and playback with the 3.2 mega pixel camera. A camera which gives some very clear and detailed images. The music player like many mobile phones now has Track Recognition. The three games came preloaded on the phone I'm using at the moment are FotoQuestFishing. A game where you dive at nine different locations snapping pictures of various tropical fish. Mini-Golf, or crazy golf as it's sometimes called, and Tennis Multiplay. A mini WTA game that includes Tournament, Freematch and Training modes and Bluetooth enabled multiplayer between mobile phones.
Speaking of Bluetooth, a Bluetooth Remote control system has been added. This allows you to use your Sony Ericsson as a remote control for other Bluetooth enabled devices. Sounds interesting but complex, I haven't tried it yet.
Everything else is pretty much the same as it was on the K750i. Video DJ, Photo DJ and Music DJ are still there and the navigation controls are identical to the K750i so you can dive straight in. For some reason though, Sony Ericsson added a torch on the K750i and have just as mysteriously removed the torch from the K800i. But that really is just niggling at what is a very good phone. A phone that surprisingly goes for anywhere between £50 and just over the £100 price. Although for £50 you're probably not going to get the 1GB memory card. The 1GB card in the K800i I'm using was purchased separately. But if you can get this phone for the £50 mark up get it, you will not be disappointed.
