Take out a new plan in an O2 store..
I have p/month;
500 texts
600 minutes (x-network, anytime) to use between tuesday and friday
2000 minutes (x-network, anytime) to use at the weekend, saturday to monday.
Free voicemail 24/7, and all the above rollover every month.
for £35. I'm sure there are better ones out there at the moment, but if you like to get savvy with a few text messages now and again, but also like the idea of calling whoever you want, when you want (c'mon, how many of you know which one of your mates are on your network or on another network?) then this isn't too bad.
Orange do the same thing but with unlimited texts and limited x-network minutes, so its up to you to choose if you like text'in or talking!
As for the phone, I have the 850i, and there are a few glitches, but its also got a lot of cool features and its the first SE phone that has dragged me away from a Nokia way of phonelife. Every other SonyEricsson phone before has seemed like a good idea at the time, then after a week or two it just drove me mad and I always went back and swapped it for a Nokia.
Headphones with the 850i are pretty good, good bass when coupled with the Megabass EQ on the Walkman player. 1gb card included with the phone, but I also have a 2gb Sony HighSpeed card so I can fit loads of music on there. I didn't get the phone to play any music or take any pictures as I have a decent digital camera and an iPod, but I'm plesantly surprised with the Walkman player and the camera does its job I guess. Never had a slide phone before cus I thought they were pretty gay, but as times change (and men get more feminine in 2000) I thought it'd be a nice change. You should go into a shop and have a play with both phones I guess. I took some pics of mine as I got bored...
http://steviepee.smugmug.com/gallery/2056096