we have used it 3 times, with reasonable success. 1-in section requested of ssr, with rooms next door to another family we were travelling with, ground floor. 2-not in section requested (but columbus day weekend), but again rooms directly across from other family. 3-boardwalk requested highest floor and got 2nd (which is really the lowest due to the shops), but during end of renovations, so wasn't too picky.
i think it's about the same success rate as when we used to just check in, i only request at ssr for the section, and all the others as high as possible. just happy to be at wdw and figure the requests (online or not) are a gift of pixie dust when they happen!