welcome to the forum. both the engine and tranny are new to the planet, iirc. however, given subaru's history, i think reliability will be the least of your concern. the sti is my daily driver and speeding tickets are a bigger concern.
i talked to this guy at Irvine Subaru who helped me buy my STi and we were talking about the tranny. The tranny is rock solid. This guy (chuck) told me he personally spoke to an ex-rally racer and subaru had him try to break the tranny. Anything he can do to break it.. from going 60mph to first gear, from 3rd gear to reverse.... This ex-rally racer (forgot his name) said that it was one of the hardest tranny to break.
Subaru also had him shift 4,000 times in one day to check the wear and tear of the gears. The tranny in the STi is verrrrrrry reliable, more than you need, unless you were to take it through the backmountain roads on dirt, jumps, hard excessive driving.. ect.