Before I comment on the doubles kit can I just say that in reference to the other rating (one star awarded) anyone that thinks they are getting a Phil & Teds buggy for £56 (price of the doubles kit) must not have done their homework. You can't even get a second hand one on ebay for that price!! Giving one star for not doing any research is a bit unfair.
Now to the doubles kit. It is easy to slip on and off and not too fiddly at all. It is worth noting that it is easier to take the seat off before taking out and placing in your baby (if you are using it in toddler and baby mode). We leave the brackets for the doubles kit in the slots in the frame as we have misplaced them before. Doing this does not interfer with folding the buggy up (nor does leaving the cocoon in the pram). Overall I have to say the doubles kit is a good sturdy piece of kit, simple to attach and our toddler son loves it as it gives him a commanding view of everything.
As a mention, the Phil & Teds buggy is also a great piece of kit (alibet ours suffers from far too many punctures!!)and I just wish that we had know better when we had our first child.
Good make and practical. I would reccommend it.