Well with the disclaimer that I've only been using my C550 extensively on T-mobile...
The C550 is mostly an excellent handset which I use to play audio and video frequently. It has a very nice QVGA display.
Some (all?) of these have problems reconnecting to GPRS. If the GPRS connection goes idle, times out or it just feels like it it will stop responding to data requests. The only way to get it to respond again is to power cycle the thing or switch into and out of flight mode.
I make heavy use of GPRS and sometimes have to do this a dozen or more times a day. I fell like stamping on it sometimes
YMMV: this might just be a T-mobile issue.
On the subject of GPRS it doesn't have a build in data counter. A major omission but you can buy third party counters.
I did use an S60 device (the old N-gage) on Orange. I was largely happy with it until I hit a hideous bug. If you fill up its internal memory it wouldn't power up and needed to be sent to a service centre to reset. Apparently newer S60s don't have this problem.