Great replay value with mutiple endings
For example, you will chase a giant sand worm reminiscent of Dune on Festina, you will fight opponents avoiding solar flares on Solar, fight a giant robotic satellite in Sector X,Y,Z, ...
I wasn't particularily impressed by the game when I bought it and only played 20 minutes one the first day. Some days later I beat the game in the story mode and found it short like most of the other people here.
Afterwards I started to replay the story taking other paths via the unlockables and it was then that I got hooked. It is only at that time that I realized that this game has huge content and has now become my number one game on the DS. It must have about 30 - 40 different missions, with each mission including 5 - 10 fights, so you will have plenty of hours of fun.
It is so addicting that it really is stealing on my sleep time, it's been two weeks that I've been playing until 1 AM in the morning each time thinking "ok, one last mission" :-)
So really try out the unlockables, you won't be disappointed