All "chips" are basically ECU tunings. Depending on the type/company this is accomplish in 1 of 2 ways. One is by literally plugging into the ECU and re-mapping the ECU, changing turbo psi, shifting, etc. The second way this is accomplished is done through a "piggy back" system. A second computer unit is plugged into the ECU (placed in its container) and it changes the codding while the car is operating. These types of system sometimes throw off error codes that can be a pain in the a$$.
BMW performance chip produces modest gains. For the power output....it's way overpriced. The thing it has going for it is that it will definitely NOT void your warranty as it is a BMW product.
There are lots of other tunes out there that produce a much better power output for a cheaper/equivalent price. However, these tunes can potentially void your warranty. You'll get a total divide from members here on the validity of that statement. Some will be all "yo man, don't worry about it. i take my system out every time i go for an oil change and they can't tell!!" others will be like, "dude, don't do it!!! once codes are thrown off those sleuth SA's can tell and they'll void you ENTIRE WARRANTY!!!" There are companies out there that back their tunes up with their own warranty (DINAN), they guarantee that is their system causes any type of problem, they fix it.
Look at some of the sponsors here, they sell most of the majors tunes; DINAN, JB3, OE Tuning , Active Autowerke, Vishnu, just to name a few.