I got the game last week, and here are my impressions:
The game is much slower than previous versions, this makes it more realistic. You have to pass around waiting for space, like in real life. Dribbling is not as easy, and its actually a challenge to maintain control of the ball. PES 5 can initially be extremely frustrating, because of the new foul system. Even small nudges are fouls, so timing a tackle is very important. As I said earlier, the ball physics are much better, and its a bit tough to keep the ball under control. The game is very similar to PES 4, with a few changes that add to the realism. But the commentary is still the same, no new game modes(except for Memorial Matches, which I don't think is great). The master league apparently has new features, but I'm yet to see them. I've been busy re-naming the teams and the transfers not yet done. But the gameplay is alot more fun, I love the new pace, its much more tactical now. Less arcadish. In PES 4, I used to win quite comfortably on six stars, but here, I'm struggling to win even on 5 stars. Overall, there are many other small tweaks in the gameplay, that one can only discover after playing the game for a while. For example, when a player gets injured, and when you put it out of play for him to get treatment, the opposition actually throw to you and give you back possession, like IRL. That's the beauty of Pro Evo.
To sum up, the main difference is the pace of the game. It plays much more slowly, I find it similar to the La Liga. Of course, the game can be fast paced too, if both teams are playing attacking football. Yesterday, I beat Barcelona 4-3, in a great game of counter-attacking football.
Jonah, I suppose you'll be disappointed that Palace aren't in the game?