It doesn't take long to finish I-Ninja, but you may still have a fun time playing through it.
You do have to redue some levels to open up new levels, but most of them are fun. And you have a choice not to do all them, just what you need to continue on. The music in the game was really well done.
There isn't much reason of replaying this game, once you finish all you may just want to put it away for a good while. Since you can finish this game fairly fast, you shouldn't end up getting bored before it ends. Its only around 10 hours or so to finish. Depending on how many extra levels you attempt. This was a nice game to play through since I usually stay clear from these types of games.