In reply to cliff shasby:
"mammut phoenix...VERY hardwearing,"
+ 1
mine are 5 years old, still going strong and I have used them for nearly all my climbing.
Disadvantages to 8mm are:
Easier cut through by sharp edges
Not as safe when climbing in a 3
Not as safe to climb soley on one rope then the other for pitches with a traverse in the middle etc
Harder to hold falls, it's worth getting a specialist small rope belay device like the buggette, I have two one for me and one for my partner. Although some of the standard modern devices do go down to 8mm they will not hold falls as well as a specialist device.
I've also heard the elasticity on the ropes decreases faster with falls, which makes sense, although I don't fall off much so not a biggy for me.