yup, just upgraded my BT line from 512kb to 1mb.
Its basically the same as 512 except all my xboxes and PC's have a more stable, faster connection.
The problem with 512kb was that it was split between 4 pcs and 2 xboxes and when you would use the line excessivly on one appliance (mainly my pc :D ) all the rest would suffer.
With 1meg the difference isn't massive, but it deffinatly noticable, and it only costs us £3 more a month so we might as-well. :D
No hassle either, i ordered 1meg, waiting about a week, then one night my internet went off, couple of minutes later it reconnected and it was 1mb :D