Aside from being mental, Jens is actually quite a bright guy who knows a lot about footballing history, economics and a load of other ****. Technically, I didn't rate him. It was those flaws that ended up with him being replaced by Alumina in 2007. He had a problem with dealing with pressure from corners and his decision-making was erratic.
But half of the battle for goalkeepers is mentality -- more so than with any outfield position. Jens was great because he genuinely thought he was ****in' brilliant even when, at times, he wasn't. That's why I think all Arsenal fans are more hopeful about Szczesny even though Fabianski has saved us more points this season. Szczesny seems to have that arrogance and aura about him.