Part of me would like the tweak-a-bility. The other part of me says "you're got an amp with 6 knobs right now... would you even know what to do with them?"
They come in four variations:
GAS-707: clean boost
GAS-808: your basic Tubescreamer, minus the compression and slight frequency cutoff
GAS-909: the 808, but with three times as much gain, as the manufacturer claims
GAS-828: two independant 808's in a single casing, of which one is switchable to 909-mode
I have the 828 (serial number 139), and it's absolutely brilliant! By the way, Lee Ritenour uses these.