He is good. The odd fuck ups he has do not come at any significantly higher rate than they do for say Curtois or any other top gk. I remember that short spell of some 5 or so games a while back he looked a tad shaky but he isn’t bad at all.
For me it has also taken a surprisingly long time to shake off the insecurity that came with watching an Arsenal gk between Lehmann and now. Years of being emotionally abused by Almunia and the likes does not easily go away, but Cech really is good. Not perfect, but good enough to win the CL and PL with if the rest of the team behaves.