I've got the Custom Code intsalled in mine, very happy overall good increase in power and torque all over the rev range and a smooth delivery. Very similar to the Revo trial I tried a few weeks before. Was saving up for the Revo and would have happily gone for it, (have used them before - mk4 Anni) but could not justify the extra money as CC have a 30% discount on at the moment, making it a bargain at £370. for around 50 bhp extra.
Had APR in my S3 too (they have a 20% offer on now and again) they were faultless too.
You can't go far wrong with any of the bigger tuners (Revo, APR, MTM, GIAC, CC) only three considerations IMHO:
1. How far do you want to travel? I would pick a reputable Dealer who is reasonably near you. Others on here can help recommend good tuners in your area.
2. how much extra oomph do you want?
3. How much do you want to spend?
Have fun