I think that the iPhone 5 is 9 months away (back to its usual summer release)
With that in mind, you can sell your iPhone 4 for £300, so it's a £200quid upgrade to get you through 9 months all told (22 quid a month). Not cheap on the face of it, but day 1 iPhone 4 people will be able to drop to a sim only £10/month deal from December 26th, saving likely ~£25/month in the process, so if you look at it from the angle of it costing what you've been used to paying... it wont cost any extra by the time the iP5 comes out. Creative logic if I ever saw it
Anyway, I got one, it's not here yet but my mates is here and browsing is a massive improvement, it feels more like desktop speeds than phone speeds now. The general feel of the thing (even swiping homescreens etc.) is much better, if you're a heavy user with OCD like tendencies for liking things to run perfectly then it's worth while as an upgrade. Most of us with a few hundred quids worth of GPUs probably fall into that category!
Siri is useful at best but it's not really that clever. It clearly listens out for key words and can't figure out variations. For example:
Me: 'Set me a reminder'
Siri: 'When for?'
Me: 'Tomorrow at 5pm'
Siri: 'Here you go'
Me: 'Change title to meet richard down the pub'
Siri: 'Here you go'
Works well, but probably takes about as long as creating a reminder. But if you try 'Remind me to meet richard down the pub at 5pm tomorrow' it completely fails. That's where the value add is with this thing, and it doesn't deliver on that front.
If you're a light user and happy with the 4 don't bother, if you're a heavy user and think the cost logic I've used above works then go for it, the day to day usage is much snappier and there will no doubt be some games etc. that take full advantage of the additional horsepower on the way.