What does the sprint booster do exactly? can it be fitted DIY and is it expensive

I suggest doing a Search. There are some strong feelings on the use of this product :)

This thread is worth a read if you want to know a little more about SprintBooster http://www.xoutpost.com/x5-e70-forum/...ntbooster.html

cheers Brian

I had it on my 2007 530 and now have it on my 2008 M5. It made a difference on both cars and I wouldn't go without it again. I should also mention that I had/have ECU remaps on both cars, which also improves throttle response. However, the SprintBooster still made/makes a huge difference on top of the ECU remap. The ECU remap still left lag on the throttle response timing. I'll put it to you this way, both cars (530 and M5) felt slow after I removed the SprintBooster when I brought the cars in for service. I couldn't believe the difference. I couldn't and wouldn't go without them.

But again, it's personal preference. Some people prefer throttle lag for safety reasons. I guess it just depends on your driving style.