Got my 123d back in april 2010 and a little while later went to cormwell, celtic tuning, where they claimed 269bhp and some ridiculous amount of torque. They almost fooled me into believing them. They didn't let me see what they were doing the whole time and once finished I took it for a test drive only for the engine management light to come on and it kept going into limp mode. So I got back to them and this problem was occuring a few times and they said it will fix itself after a few more miles on the clock. I took it back and done 12,000 miles more but it never stopped going into limp mode, which almost destroyed my bmw warranty which is an '10 plate. Then they had he cheek to say they can't do anything about it, bearing in mind I am in london. Anyhow, after ages I got them down to remove it.
Left it for a few months then went to the Bimmer forum and got the contact for Jason at bwchiptune and paid him a visit. Unike celtictuning, Jason let me see the whole thing and started the tuning process and within a few hours it was done. However one thing I would like to point out, when Jason took out my ECU we realised the **** up of celtictuning and why they didn't want to see me again. It is because cetictuning had burnt out one of the tracking on the ECU and all they had done to solve it was by soldering a peice of wire in there to replace the burnt tracking and left it exposed. Anyhow Jason being a gentlemen wrapped some insulation tape and continued with the process. (have pictures to prove this and will upload soon). I recommend Jason at bwchiptune as an excellent service. My 123d has had an immense power surge without going into imp mode. If anyone out there has any bmw cars, I highly recommend Jason and the lads at bwchiptune. Also just to add Jason is an ex-bmw technician and has all the professional gadgets plus a rolling road at his place.
** Do not go near cetic tuning, there absolute scumbags**