Here's a good video if you're interested in an eye test for Koskinen.
Honestly, hilites aren't good to judge a goalie on because the saves that make a highlight reel means there was a breakdown of some sort and the goalie wasn't in prime position. With the exception of breakaways and two on ones obviously.
From this video I don't see too much to be concerned about. Definitely doesn't have the mobility issues that some have trumpeted. This isn't a Gustavsson situation where the guy literally moved like a giraffe on skates. He's got good quick movements, good ready stance for his frame that doesn't reveal holes through the body. Typical high glove position for Scandinavian goalies. Seems to be very active with it as well making a lot of baseball style catches.
Things that stood out to me where he will definitely need to improve is his post integration and his depth management. He rarely goes out much further than the very tip of his crease. Even with his size, I'd like to see him play a bit more aggressive. The post management will be huge as NHL players are very good at jamming the net and shooting from bad angles.
All in all, I'm interested to see what he has to offer. He already looks like an upgrade to Montoya and brossoit. I doubt he will take manynstarts away from Talbot though. And definitely don't see enough yet for him to supplant Talbot.
Then again, you don't really need to find the next Tim Thomas in order to win the cup. As this years playoffs have shown, your goalie just has to be mentally strong enough not to let in the squeakers. Every goalie in the playoffs right now has let in a touchdown in one game (maybe except for fleury, can't recall ATM). The position is much more dependant on systems and team play in front of it than in ages past. If Koskinen has that kind of mental fortitude then he can definitely make a career for himself in the NHL.