I went from a 2010 WRX to a 2013 STI and I'm not sure what the differences are between the '10 WRX and the '11 WRX's are (other than the obvious body and exhaust changes)' but I couldn't be happier. The handling difference is night and day. This STI feels like a sports car compared to my WRX. The 6 speed is so much more solid and precise feeling compared to the WRX. Stock power feels about the same or a little less, but the stage 1 OTS Cobb flash has been amazing. I don't understand why some people say they didn't notice much of a difference after flashing on their STI's. I do know Cobb has made some tweaks and managed to get a bit more out of the new tunes. It now feels much more powerful than my WRX was. Oh, and you can turn hill start assist off (though I think I read that you might be able to do this now on the '13 WRX's). The breaks feel quite a bit stronger in the STI, also, but I guess that's obvious with the Brembos.
The one and only con, I'd say, is the gas mileage so far has been about 5mpg worse, even when I'm driving it very "nicely."
This is me comparing to a 2010 WRX, so I guess it may be a bit different than you would experience coming from a 2011.