I'm with Chrome and Dozer. I'm not smart enough to use it, so it's worthless for me. I'd prefer they fix the damned clocks. Last night, as I was falling farther and farther behind, I watched the clock that shows just how poorly I was doing. At one checkpoint, I'm 41 seconds behind, then I'm 39 seconds behind and then 42 seconds behind. Sometimes it got tired of showing how badly I was doing and didn't show anything at all.
I don't recall this being a problem with GT3, GT4 or Prologue, but it happens frequently to me in GT5.