I'm not aware of any transmission issues specific to the 2011 model year -- links? The WRX transmission is Subaru's 5MT and as far as I know was mostly the same from 2008 or so (if not even earlier, barring diff ratio changes) until 2014. It's got a rod linkage for its shifter and has a reputation for being a bit rough but I think that gives it character
I've got a 2011 with ~50k miles on it that I bought new. It's a bit of a rough car in general in terms of noises and things like squeaks & rattles, but drive nicely enough (a bit softer than e.g. the Mazdaspeed 3), has plenty of power, and handles well. Mine has the Subaru short throw shift kit which give it a nice mechanical feel. It is not a sports car shifter.
In terms of reliability, I haven't had any problems with mine and would count it as being as good as anything else I've owned or cared for. Some are obsessive about every little thing such as interior squeaks, small amounts of oil consumption (mind burns less than the Civic it replaced, about a quart in 4k miles), etc. and if you could that kind of thing against "reliability" then, well .. it still may not be worse than whatever other misc car.
The interior on mine is looking a little shabbier than new but it's a dog car and driven in dusty areas and most of that is probably that it just needs a good cleaning.
The bad:
- The windshield seems fragile in terms of chips, but hard to do an AB comparison and I might just be unlucky. There is a quarry nearby. No cracks though.
- The paint will scratch if you look at it wrong. Scratch-X and Swirl-X work decently on it though.
- With all Subarus if you puncture a tire beyond repair you may be looking at 4 replacements instead of 2.
Overall I love this car.
I *would* be wary of buying a used one, especially with mods. Evaluate the owner.