For some reason i enjoyed this game less than the original. Here's what I enjoyed.
The leveling up of the weapons. If you used weapons enough, they leveled up making them stronger. Space battles. While these were easy, they provided a change to the running/shooting gameplay.
The Arena battles and racing were good but got quite tiresome near the end of the game.
I did not like the actual gameplay though. At the end of the game enemies have annoying amounts of health and you just have to chew slowly through them. The last two planets I just chucked on a shield and jogged past all the enemies. I was bored and wanted to see the outcome which was disapointing.
I would suggest buying Jak II to be honest. People say you should get both but this isn't as good.