Better than WE2007
Last year's WE-PES2007 received great reviews for the PSP platform. PES2008 has good reviews too, but they usually don't recommend it as an upgrade. I think PES2008 contains some important improvements, so I'll just list the differences and I'll let you be the judge of whether it's worth upgrading or not.

Neutral: Player models/animations are unchanged. Same stadium as before, no extra stadiums.

Negative: Longer load times than before

Positive: Player responsiveness is improved and dribbling works nicer (players keep the ball closer to their feet). Slowdowns are rarer and not as significant. Master League now includes player experience which was missing in the previous version. More national teams are available.

Great soccer game.
The game in general is very good. Some bad issues are. Long loading times, (Long is 15-20 seg), Only one stadium. No players entrance scene, No real team names for some teams, No online option and No audio commentarist, that is boring. But the GOOD and most important is the play mode is very, very, very real. The bounce of the ball and the player's movement are awesome.

Just the same game from last year
I was very disappointed to see that PES 2008 is basically the same game from last year. Even some portions are worse when it comes to gameplay. First off the ball is extremely bouncy. It is bouncing off almost everytime in one on one tackles. Secondly, it has become extremely impossible to score, even at Regular(mid) level.

As far as the improvements, there are a couple of additional team licences, some players got facelifts and that's all. I don't play master league but people say that it has been improved. And the menu is somewhat new.

Other than that I think it is still the same old PES maybe even worse in gameplay. There is only one stadium(everything's same, from textures to the cutscene). There is no commentary.

If you have PES 2007 from last year, I would say just stick with that.