I've been replaying the Mario & Luigi series, and now I'm remembering what made them stand out in my mind as great. All three in the series have there ups and downs ; for example, Superstar Saga's battle system seems a little slow compared to the other 2, but the music is much, much more memorable and fun to listen to, compared to Bowser's Inside Story, which has a fast and fun battle system but somewhat annoying music.
All Mario RPGs are great to me because the action element added into battles and exploring breaks the monotony of turned based battles. Plus, there's no random encounters, always a plus.
If you haven't played a Mario RPG, or just want to play an RPG with a little extra, you should definitely pick up Mario & Luigi : Superstar Saga (GBA), Mario & Luigi : Partners in Time, and/or Mario and Luigi : Bowser's Inside Story (NDS).
By the way, I totally agree that the Blue Shell sucks ass.