Little Big Planet is a great game considering it is a portable version of its PS3 predecessor.
The story mode is pretty short. There are no new innovations to gameplay but they weren't really necessary. The playtime for story mode is roughly 6 hours (add more if you collect all prize bubbles) but it is still a pleasant experience and it will provide you with tools to create levels.
Creating your own levels is where this game shines at. Having the ability to download community created levels adds a lot of replay value.
One of my major gripes about this game is the loading times. Just like in the PS3 version, this game takes its time loading. While loads screen are not a frequent occurrence, it gets kind of boring specially if you can only play one level at a time and then have to turn the PSP off (since most of the loading takes place when launching the game).
The other little issue I found is that sometimes switching panes doesn't work that effectively. If you've played the PS3 version, then you kind of know how it works, if not, well, it'll take some time to get used to it.
Besides a some tiny minor issues, this game is great as it allows you to download community created content which will greatly expand LBP's experience.