I have been using my new Markins Q20 ball head for a few weeks now and have to say I am impressed. I will not go into details like weight, height, load...etc... but just will say it is an amazing piece of engineering. Having had already heads from Benro and Manfrotto this is a class above. I can use it with the 500 f4 IS alone or with the sidekick without worry, perfect fit with my Gitzo 3540LS (review later maybe ;) )
Here is a picture, sexy or what?
7D, 500 f4, 1.4TC, sidekick and Q20
My tip: if you are travelling to USA or know someone how is, get them to bring you one back, saved me 150€, thanks Ronny!