Car and Driver has a long term road test of the 2008 Subaru WRX STI. It looks like they got a bunch of mechanical issues with the car. The review is pretty extensive and is worth a read.
“The first few months with the STI were smooth sailing. [...] Around the 11,000-mile mark, second gear began repeatedly popping out of its engagement, and into service the STI went.”
“Engine problems cropped up with just 23,000 miles on the clock. [...] An air-fuel-ratio sensor was initially suspected and subsequently replaced, but the new sensor proved no fix. Finally, after months of trying to track down the unhappy ghost in our engine bay, a new engine control unit was installed, curing the problem. [...] It took so long to identify the issue that Subaru asked the dealer to do a compression check on all cylinders, to make sure driving the car with the surging issue hadn’t caused it to go out of spec. As it turned out, the compression was indeed out of whack, so a new short block (which really means most of the engine beyond the cylinder heads) was installed at 32,066 miles.”
“[...] when the car was functioning properly, it was an extremely capable and fun-to-drive machine.”
“The all-wheel-drive system proved fantastic during the winter months and the suspension provides a much more civilized ride than that of the Mitsubishi Lancer Evolution.”