I traded my 60,000 mile 2004 WRX STI for my 8,000 Mile (At the time) BRZ. The absolute only downfall is less power. And quite frankly, I don't even miss it at this point as the BRZ almost has more useable power (A lot less drama to get up to speed we'll say, at a normal rate). Now, a WRX is going to be more comfortable and quieter then a 04 STI (04 STI's had no sound deadening...etc) but really, I never purchased my STI for BMW Fit and Finish. My STI achieved 21 MPG no matter how I drove it. Drive like an asshole? 21. Drive like a Grandma? 21. My BRZ rewards me with 28 MPG on my mixed commute. On a 28 Mile drive one way to work, that's a big deal.
Once I figured out the BRZ's powerband I never looked back.
-Sam