We used the latch loc when kiddo was born. It's actually what got me interested in CPS . I couldn't figure out how to get it tight in the center of my car. Turns out it wasn't me, it was the seat. It was just barely acceptable outboard. The tech said she'd had trouble with it with seatbelt installs. The Rigid LATCH was a breeze to use in my mom's car.
I also had trouble tightening it--very sticky, plus the handle was hard to move, and it was easy to get it mostly but not all the way snapped into the base. Hmm, yeah, didn't like it much. Plus the puzzle buckle!
I think the cons outweigh the pros especially for the flex loc model, but it wasn't horrible.