What Andy said.
A friend of mine has the Carsoft and we played a bit with it. Seems decent, but then we discovered a few bugs - it throws some errors concerning the oil pressure if I'm not mistaken. Also, it cannot access all the modules. Then, later on we found out that it could mess your adaptation values for good (I know you can store the original/OEM, but God knows how these knock-off Chinese software really work). This kinda scared my friend from using it, so now he is talking me into buying a GT-1 (together of course). This I might give a try.
Also, what Donna suggested. There might be some "decent" knock-offs which will not mess your car up (Carsoft $$ < GT-1 $$)
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39, e53, e46 & Tribby