Current GTI owner, also own an FR-S. I've had the FR-S for about a year now (10k miles) so I figure this would be a good time to update. It's been fairly reliable for a first-year production car. Only warranty work was a dealer ECU reflash for the cam timing issue, which was done in about an hour. No problems since then.
I've had a busy summer with work (and getting married and buying a house), so most of the miles have been city driving, but I've taken it out to a handful of autocrosses. Only mods are tires/wheels, but as mentioned, that makes a big difference on this car. The usual Z2 SS or Rivals are good for a 3 second improvement over a 30-45 second course. As the reviews say, the balance on this car is very good w stock suspension. Even on sticky tires, you can adjust the car's attitude in corners using the throttle (power oversteer) or brakes (trail braking). The car also gives excellent feedback, so you can ride that line between grip and slip easily. The steering is quick/direct (2.5 turns L-to-L), so you leave your hands at 9-3 for most of the course. Fun car to drive at the limit.
I'm surprised at the comments about the FR-S/BRZ being slow at the track. Mod-for-mod, these cars are proving to be comparable in laptimes to S2000's. Guys who've extensively tracked and data-logged both show the Twins with the advantage in cornering speed at the bolt-ons level. These cars have also smashed (by 2-3 seconds) TTD course records at tracks like Sebring, Roebling Road and Gingerman with limited mods (coilovers, tires, pads, tune).
I'm hoping to get this car out to VIR or SP a few times before the year ends. I probably won't have time to do any additional mods beyond pads/fluid and camber bolts up front, but based on how it does in autocross, I think the stock suspension will be fine for the track on street tires.
The way I see it, the BRZ/FR-S is a great alternative for someone who doesn't want to deal with the hassle of putting in a roll cage in an S2000 just to be able to run track days with it. These are lightweight cars you can drive on a daily basis, and track/autocross with on weekends. The biggest downside to me is the power delivery (specifically the torque dip), but it looks like a UEL header + tune solves that issue completely. I'll probably look into that for next year.
BTW the GTI is doing great too, just a very different car built for a different purpose. Hard to make a direct comparison.