After having lived with Sony Ericsson K810i for almost one month, I have mixed feelings about the phone. Obviously, it is an improvement over my previous K750i. The screen is bigger and has better resolution. The camera is improved with 3.2 Megapixels, the keyboards is better, but I found the speakers low in volume. Of course, with a handsfree you will not have this problem, but then again, the K750i was better in this department.
The K810i is not a "new" phone, but rather an improvement on the well known K800i. The keyboard is different, there are some cosmetic changes also but nothing fancy. My guess is that if you are already a K800i user, I see no major reason why you should go for the K810i, apart from the fact that it is better looking. The upcoming K850i seems a more logical choice, although it has not hit the market yet and is due for release in Q4 of 2007.
In any case, I am a big fan of Sony Ericsson mobile phones. The latest K810i is without a doubt a successful model with some minor issues. I hope that with a new firmware upgrade most of them will be solved. At a retail price of €360, it is not a cheap phone. I see no major competition from Nokia phones, since with similar features you are looking at much higher prices than the K810i.