I personally find i can reach orgasm quicker when im on my period and i feel loads better after because i relax so much (down there) which in turn makes me want to do it again :P
I also find it better during the first three days (mine last 4/5days) and i hardly ever get pains anymore, but it may be a mixture of doing that and the pill which has reduced them
Not everyone's cup of tea though, some find it too gross doing anything when they're 'on', i used to until i became so desperate i thought fuck it and now i dont care!
And like RHH said, it helps make the period end quicker when u do it which i find is a major bonus ;) and they're right about clots
I tried one month doing it at least once each day and i was 'on' for nearly 4 days. Next month i didnt do it at all (very frustrated by the end!) and it lasted nearly 6 days. So now i try and make sure i do something, but that was me personally