In reply to DemolitionRed:
> I think the reason many people have problems with half ropes is, they don't clip into the pro in the right sequence and if you make that mistake your going to get tangles galore.
Would you care to explain how clipping sequence produces rope twisting? Why isn''t this a problem with all diameters? For that matter, why isn't it a problem with single ropes, since after all one strand of a half rope pair is just a single rope clipped to some pro.
One thing a lot of us do to mitigate twisting is for the belayer not to tie in on the first pitch and have the leader take up almost all the rope before the belayer does tie in. This gives some of the twists a chance to work themselves out before being ``locked in'' when the second ties in.
Many rappel devices induce some twisting. If the ends are hanging free, this is not so much of an issue, but if the ends are on a ledge or the ground, the twists have nowhere to go and start climbing back up the rope towards the rappeller. Not much you can do about this, but it does help for the first person down to re-pile the rope so as to get rid of the existing twists before the next person down adds more.
Tangling is a different issue from twisting (although the two can certainly occur together) and is primarily a question of how ropes are stacked. If there is enough room on the belay ledge to pile ropes, this seems to me to always be better than flaking the ropes over a tie in; I've never had tangling issues with piled ropes. Ropes flaked over a tie-in can tangle easily and quite badly when loops that are being paid out capture other loops. There are well-known stacking techniques to avoid this, but they can be difficult to implement while managing a belay properly.
My impression is that 8mm halfs tangle and twist significantly more than 8.5mm halfs.