It doesn't look like a bad deal for you. I think Prospal looks good spuratically and you are losing a goalie who is doing pretty well right now. But I think right now is the operative part of that sentence. This is the way I break it down.
Ovie for Gaborik. No brainer Ovie wins (but really not by as much as you would think) I just think it's funny when people say ya do it because the guy is going to get injured. Don't count on him getting injured he is going to have a great season. But you get Ovie which is good for you.
Prospal for Kopitar. As much as people would like to believe that Kopitar is a fluke, Prospal's hot start is probably flukier than him so you are losing one of the best C. Luckily C are easily replaced with some decent talent.
Rinne for Johnson. If you have a couple solid goalies and one bum defenseman then this is a good trade for you.
Overall I think that if you have another C who is producing a point per game and you have another RW who is putting up decent numbers, this is a great trade for you.
I just lost my train of thought. I am watching the Hawks game and Hossa and Sharp just scored back-to-back SH goals on the same PP for the Sharks. Oh ya! Take the trade but don't think you just broke the bank.