It, to me, was the Episode I of Indy films... That puts it as low as possible in my Indy rankings. I enjoyed it for what it was, but I was disappointed it wasn't as good as TLC or TOD to me... I didn't expect Raiders level of greatness at all.
I don't feel Mutt ruined the film, but I don't feel Mutt added to it either. He was very unnecessary, and did some really dumb **** in it. The vine-swinging was comically bad, and I felt hurt that sequence. His sword fighting/fencing BS was bad, and a barely veiled attempt at giving him an iconic melee weapon ala Indy's whip. The ending was ambiguous on purpose incase the movie was a smashing success, for Mutt's "taking over" to be easily accepted, but also incase they want to do another one with Ford.
I felt Mutt should've been much more "Short Round" than "New Star", and just a secondary character.
Ford was fantastic. He made even the ****** parts good. That's why Harrison Ford was the highest paid actor in 2008 (that, and he's incredibly intelligent to get a part of the Box Office too

). If there were "Indy" moments in it, he made them. Neeson and McGregor were great in Episode I... Doesn't mean it didn't suck, especially compared to all the others.
Part of me is hopeful Lucas/Speilberg can improve greatly upon the Indy franchise if they do another film with Ford, and don't go down this road again, or at least tame it greatly. Mutt isn't a bad character idea, he was just terribly executed in KotCS.
However, I probably would take another not-so-great film just to get (hopefully) many new toys from the original 3 films.

I'm a whore like that.