Subaru's flat four is a horrible engine IMHO. Fuel efficiency is poor, they have cheap ass cast pistons, and they run their engines very lean during boost to pass emissions. Google 2011 Wrx/Sti dyno chart and you'll see what a poor power curve the factory tune delivers.
I had a 2009 WRX and I found the flat four to be a pretty good engine, actually. The power curve might be poor to some because it uses a larger turbo which has a bit more lag. In daily driving, this was never an issue and it was only noticed during more spirited driving, but that was part of the reason why such cars tend to be more fun if you can shift the gears yourself; you can keep the engine in the "sweet spot" of the power band. Also, I frequented the NASIOC forums when I had that car and people didn't really complain about it running lean unless you were to add an intake. Pistons were never an issue unless people wanted to get up to "stage 3" power levels.
I only decided to get rid of it for styling reasons and practicality reasons. Gas mileage wasn't the best compared to the GTI, wasn't comfortable on trips longer than a few hours (which I do relatively frequently, and the exterior was oh-so-ugly, but the wide-body 2011s actually look pretty good to me).
There are many times I wish I kept it, like whenever I want to go fast or if I'm in a tunnel. That car with a catback exhaust was niiiice.
Boxer engines drink gas. My friend had an RX8. That shit had him visiting the pump all day.
The RX-8 has a rotary engine, and yes, the current-gen rotary engine does consume a lot of gas (and oil). The boxer engines may appear to consume a lot of fuel because they also have to deal with a lot of mechanical drag with the AWD setups that Subaru offers. There is always room for improvement though, and GunKata pointed that out.
The RX-8 has a rotary engine and they are known to have high fuel efficiency.
I'm under the impression that they're known for low fuel efficiency and high oil consumption.
Back to the topic at hand:
If you're considering these cars, I'd wait until the Toyobaru comes out so you can test them back to back. I'm not sure if I'd go for the Scion/Toyota/Subaru quite yet because as others have mentioned, it will be a first-generation car. Also, the poor Scion will likely be associated with "ricers" and your other typical Scion crowd. I'd wait and see if Subaru comes out with a turbocharged performance version (because let's face it, I'd expect it to have unequal length exhaust headers like its other turbocharged engines and it should therefore have that beautiful-sounding boxer rumble).