I've got the "ERT". What you need to keep in mind when dealing w/99% of the s/c out there in the states:
it's the tuning. They all sell the same product.
There are two s/c choices - Powerdyne and Vortech
You can pick one up a used one the cheap (approx 50% of paid value) sometimes in the 'for sale' section (also use 'want to buy') and EBAY. I just went to the WTB section and some guy is selling a turbo on the cheap.
Here's a comparo of the big four s/c's:
http://home.attbi.com/~kbleck/
Here's some turbo sites (believe theyre both overseas but have never heard anything negative about service and followup)
http://www.gsmmotoren.com/gsm2.swf http://www.renn-sport.net/BMW/