I liked Rikku and I believe she was neither overrated nor underrated. The characters in FFX were rather transparent, you could tell what kind of person they were shortly after being introduced to them so there weren't any surprises there. As for her combat skills, she had more going for her then simply having access to the thieving skills off the bat, she also had access to underwater areas and combat along with Wakka and Tidus. It wasn't necessary to level all three of them up, but without the support of the land based characters it was certainly helpful to have them strong. I taught her Thundara and then Thundaga as soon as possible to exploit the fact that she was one of the underwater characters.

Now, having said all that, I prefer Yuna over Rikku, over every other character for that matter. But it just comes down to personal taste, If you complete the sphere grid then the characters are almost completely identacle.