Here's a contrarian view...
I found the Gitzo's mostly overpriced, and the lighter ones not quite tall enough for me. I use an Eos-1Ds II, often with the 70-200 f:2.8 IS or 100-400 IS lenses. I've found through experience that using any center column elevation causes instant instability, so the legs have to be long enough. I'm 6'2" tall, and find that landscape photos taken on any kind of slope require a taller tripod than you would think, at least if you don't want to be squatting all day; my middle-aged knees don't tolerate that anymore.
I ended up getting a Velbon "NeoCarmagne" 830. It's rock-solid, reasonably light carbon fiber construction, and the legs alone go well above 6'8". Its collar locks IMHO are easier to handle and much less likely to jam than Gitzo's. It also goes down to ground level, once you take a hacksaw to the redundant center column.
This tripod is $599, or £309. A little steep for legs alone, but it'll last forever.