I read a lot of reviews of this phone on the internet before I bought it, and it came off slightly worse than the similar w810i, which is about a quid cheaper each month. However, the w810i is not a 3G phone, and, as far as I could ascertain, does not have a standard headphone adapter socket, meaning that only the Sony Ericsson headphones provided could be used. When trying out the w810i in the shop, in comparison to the w850, the keypad seemed basic, and did not deliver as much feedback (in the form of a positive 'click' sensation) when the keys were pressed.
The main criticisms levelled at the w850i seem to be directed at the main keypad, which was considered to be too stiff. I agree that this is a slight fault with the phone, but it is something you very rapidly get used to, as with any quirk on a new phone. The battery life was criticised in a previous review, but I have found it to be similar to the excellent battery life of my previous phone (W750i). I would suggest that music can be played through the headphones for around 8-12 hours at a reasonable volume on a single charge. This is more than adequate, an similar to many mp3 players on the market. Another criticism of this phone was that the keypad was too close to the main body of the phone; this has simply not been a problem with my w850i, although I can see that if an unusually fat thumb was used, this might be the case. The 2mp camera is not as good as in either my previous phone, the W750i, or in the W810i, but is still a reasonable camera for outdoor shots and static indoor shots. However, it is not something that would produce good shots in, for example, a club. If you are considering buying this phone for the camera, don't, period.
However, the real benefit of this phone is the music software. The phone comes bundled with a 1gb card, which is fine if you have 10-15 albums you want to listen to, but I have purchased a 4gb card (at present an 8gb card is £100 at play.com). This will enable me to store my entire music collection (in mp3 format). The software on the phone is beautiful, offering a similar 'browse by...' function to the iPod. You can browse by artist, album, track, and create and play 'playlists'. The software is quick and good quality, and sound quality with the bundled headphones is superb. The headphone lead comprises a bit which plugs into the phone, which has a standard 3.5mm headphone jack at the end; the second part of the headphones plugs into this jack and has 3 different types of rubber earpiece; the best being 'earplug' style, blocking out much ambient noise. The 3.5mm connector means that you can play music direct to external speakers, with the appropriate connector (available for most devices for about £2-5). In this manner, I can connect my phone to my hi-fi and it plays the same quality sound as if I was streaming the music from my PC.
The music software for the PC isn't quite as impressive, relying, as it does, on a USB 1.1 connection (which means songs take about 15 seconds each to load to the phone on the maximum 192kbps setting). However, all you have to do is select the music you want on the phone in the program, then select 'transfer to phone'. The transfer process can continue unsupervised, so, much as this slow connection is annoying, it shouldn't be a problem if you can just leave the music to upload and come back 30 mins later (for approx 1gb transfer). The software does, however, intuitively recognise track name, artist and album, which are then displayed on the phone. It did this without any help from myself.
This phone is good-looking and smooth to use the intuitive user interface, I'm glad I bought it instead of the w810i, as the music software and feel of the phone is superior, and I feel that the phone will still feel modern a year and a half from now when my contract runs out. I strongly recommend the w850i to anyone who wants a phone for the purpose of playing music.