After playing for 3 hours pes 2008 on ps3 here are my thoughts:
It is certainly an improvement over pes6 but has flaws. There are slowdowns in every match which are annoying but I think you will eventually get used to them. What worries me is the goalkeepers. I was on a one-on-one vs casillas with tevez and the shot went between his legs and then in the back of the net. It thought "ok, 1 in a million". Then played vs juventus and I made a cross with ronaldo and buffon, who had mase some superhuman saves before, completely misjudged his exit and the ball went on to giggs for a super easy header.
Also, I scored the very first free kick I was given with ronaldo and I am very average with free kicks on pes6 and I had plenty of other free kicks hit the post. So I'd say they made free kicks easier which is nice, I think.
Bottom line, my opinion is that if you alreasy got ps3, then get it. If not, then I think it's not worth buying ps3 just for pes2008 and personally I won't buy ps3 until a very good price drop.