Definitely a ton of fun. I only played the first one until about level 15 (Unlock Ronin) before life got in the way and it was put down. I recently got Untold and it has been fun, completely different storyline too.
The game is NOT easy, especially if you don't do some grinding you will quickly find yourself deep into higher leveled territory than you should be, leading to game overs. Save often.
They are hard as hell, I'd say start with the first game in the series since the 3DS is backwards compatible, might take some time to find a copy though. I tried the demo of 4 and it was really quite challenging even on normal mode.