The way that I view keepers is like this...
How many times this season has he pulled off a save where you said "WOW, he really bailed us out there!!!"
Not many that I can recall, not nearly as many as the keepers that have played against us. (naturally we shoot more I know) I honestly struggle to recall even once where the defense was beaten and Szczesny saved their ass. I can remember the defense letting him down lots of times though. And that's why he has one of the lower save percentages in the league at under 66%. I'm not saying that he should do this regularly, but the good keepers will make unexpected saves from time to time, ours don't very often at all.
Not even considering his errors, I don't view him as an above average keeper right now. His command of the defense appears to be poor as much as they're out of position, not attacking corners or set plays, he should be getting them in position, letting them know when a runner is coming from behind and so on. That could be because he doesn't communicate well, they don't listen or they're just not that good, I don't know for sure. He doesn't attack crosses well at all, just go back and watch what Mandanda did to us every time we put a ball in if you want to see how a good keeper deals with them. And then there are the questionable shot stopping situations.
The good thing is, he's really young and has time to improve. I'm just tired of having a squad full of potential and a couple of finished articles.