In reply to chaude:
I've had a joker which i've paired up with my Beal Cobra for about 2 years now and always been pretty pleased with it. It came really handy on a trip to La Pedriza where weight was an issue on long approaches and where we would be climbing some pitches on a single and others on half ropes. The joker was perfect for this and it allowed quite a bit of flexibility.
I've also used it in winter conditions and the dry treatment works really well.
I use this combination on almost a daily basis when trad climbing on Gib limestone and i'm pretty impressed with the durability of both these ropes, though recently i'd been thinking it would probably be best to just have two Beal Cobras.
I really don't think i've got as much out of the joker as was intended (except on that trip to Pedriza).
I'd recommend it if you're doing some really hardcore sport climbing or some really lightweight/specialist stuff where the flexibility of switching between half, twin and single is necessary.
For general day to day sport climbing just get a single rope.