I am running a rear hope mono trials. Its petty good but not great.
I hold for most moves, but as soon as your attention slips, so does the brake.
You also get the spoke flex issues, a maggie is much has a much more confident feel when on the back wheel.
To round up:
Get the disk if your not so much into big moves, it will slip. If you just do small things or are a beginner, then its fine.
PS I will probably stick the disk on for muddy comps, thats possibly the only place where a disk would beat a maggie.