For a start, it's big, and very heavy (specs say 12kg). Its full title is the Sigma 350-1200mm f/11 APO (yes, the widest aperture at any focal length is f/11), and it's an older manual focus. To say it's a fairly specialised lens would be a bit of an understatement, as handholding is, shall we say, challenging, and even a standard tripod is going to allow some movement. You'd be best to have this either on a bean bag or two, or on two tripods, one for the lens, one for the body.
Handholding is also out of the question as it would be impossible to focus too.
It's about the same size as a small bazooka. :-)
Here's James giving you a good sense of scale.
We took the lens outside the shop, and selected a convenient target to see how much magnification this monster was going to give us. Looking down the mall we picked the menu board outside Starbucks, focused, then waited for the camera shake to stop - this tripod was clearly and unsurprisingly not up to the job. Focusing was best achieved using live-view with a magnified view, but success was attainable thought the viewfinder too, it was just that bit darker than you're used to with that small max aperture.
The resulting shot. The lens also has a matched 1.4x convertor with it, which brings the effective max focal length up to 1680mm, albeit at f/16.
This shot shows the view with a 50mm lens on a Canon 5D111. The board we were shooting is right in the middle of the frame.
If you fancy this lens, it's available for the relative bargain price of £1,500 from Cameraworld, full details here. I reckon it would be good for astronomical shots (would be lovely to try the moon with it), wildlife from a static position, maybe a purpose-built hide, or for spying on the neighbours at the other end of the street. ;-)
Many thanks to Jordy at Cameraworld for the photos. :-)
EDIT: I understand this lens is now sold!