It'd be okay, but the battery life is still not going to be good if you're using gps or wifi, same as all the other smartphones.
I wouldn't buy one on an 18 month contract personally, it's going to look really dated in 18 months, as almost all application development moves to the more modern smartphone platforms. Even Nokia stop developing new applications for their phones in approximately 2.5 years (with their new maps stuff, they've stopped supporting the N95, which only came out a year or less before the 5800.)
Unless you use zillions of minutes, you should be able to get it for <£20 a month – you can get the htc desire for less than that on Three, and that is a brand new phone that is worth about 3 times as much, rather than an end of line thing.
The Nokia X6 feels horrible and cheap.
Personally, I'd recommend an HTC Desire, they are great.
Joe