I had mine remap'd 2 years ago at Viezu :
http://www.mbclub.co.uk/forums/engine/114747-any-remap-group-buys-midlands-south-wales-8.html
It was the best thing I did, mine is a Delphi non-blue efficiency engine OM646, 167hp, hence the economy wasn't great before and even now I get maybe 2mpg more around town. I was told at the time, the Delphi ECU's take a long time to adjust and the power will ramp up over time and that certainly was the case, my W204 C220 CDI Sport will do close to 7.2s 0-60 which is definitely a lot quicker and does pull you into the seat more. One thing I will say, you do get used to it, everyone I take with me says my car is very quick and dashcommand regularly tells me i'm hitting 210-219hp, which is at least +43hp. The power delivery is a lot more consistent when accelerating as opposed to rocking forwards and backwards during gear changes.
Biggest issue I have with the car is the 5sp Auto and stupid gear paddles which are next to useless, especially when the car is reasonably well noise insulated, so you can't really tell when to change gear.
I will say the upgrade was a nightmare in the respect that the ECU had to be removed and the chips physically reprogrammed as opposed to going through the OBD, but I wouldn't let that stop me, but I would expect to pay more if I went that route again.
For W203's I believe those are done via the OBD, hence you could be in an out in 20 mins on a good day. The OM646 148hp was also used in the latter C-Class, so you're experience may be the same as mine.
