I had a MIM Jazz fretless. There were zero issues with it and it ended up in regular rotation at the local pro studio. Nice finish, excellent setup, great sound. Alas, I'm a Precision guy so it had to move.
Although my current fretless is an unlined Godin A5 (it has markers inlaid into the edge of the fingerboard) I really appreciated the lines on the Jazz. It was very easy to play in tune.
And for the guys telling you that quality is hit or miss in MIM Fenders, unless they give specifics they are most likely repeating something they've heard. I have owned three MIM basses so far, and each one has been a gem. The only one I traded was the Jazz, and the new owner has been delighted with it.