I tried Xgl today – and whist it looks nice and supports my twinview setup (shocking!) I coudn’t get on with it. The reason I couldn’t get on with is it because you need to replace kwin with something else, as kwin doesn’t have support for it yet.
So… I went onto #kde and asked if there was going to be support for it in KDE4, apparently there will be, but this answer isn’t offical. Some kind person (PhilRod) pointed me to a digest , which in turn pointed me to the KDE svn repos showing commits to kwin that appear to be relevent. I’ll try to figure out who lunakl is, and ask him for an offical answer.
Update: Having mailed lunakl (Lobus Lunak) he pointed me to an article on On KWin wobbling and such stuff over on kdedevlopers.org. As it turns out, KDE is in need of graphics developers, and this was dgged about last month. So, it’s planned… but some code monkeys with ubergraphical skills are needed.