Ziptuning doesn't have a dyno, sort of testbench on which they measure your cars original HP and torque, then tune the chip, and afterwards show you the new attained power.
(example from 535D :
http://www.kempower.be/NEWS/E60_61_535d_272hp_st1.jpg)
So at ziptuning's they just replace/rewrite the original ECU and don't bother to test it, so how would you know if you get what they promise you? Of course that's cheaper then real engine tuning, where they make a new ECU program specifically for your engine (no 2 engines are exactly the same). I'm going to have my 320D done at Kempower, I've talked to lots of people who have experience with this company and they are all very satisfied to say the least.
http://www.kempower.be
http://www.kempower.be/whatis.htm