I'm testing the latest version of the full-frame Sigma 120-300mm f/2.8 zoom. To state the obvious, it combines some of the capabilities of the 70-200mm and 300mm f/2.8 lenses in one lens but without any real penalty in size or weight over the latter (if Sigma had managed to produce a 70-300mm f/2.8 it would be considerably larger and more expensive). At $3,199 / £1,999.95 it's around half the price of the 300mm f/2.8 primes from Canon or Nikon and is certainly an intriguing proposition. Strangely, Sigma don't go to great lengths to promote the weatherproofing, their press material says it's sealed but there's no mention of it after that. And, I should point out that the lens mount doesn't have a seal, which is an odd omission at this level.
I've not taken enough pictures yet to formulate any conclusions on image quality but I've some initial impressions on the build. A large heavy lens like this must be equipped with a decent tripod bracket and, thankfully this lens is, by and large. However, it has just the one (1/4-inch) tripod socket in the foot. I would have preferred to see two for secure fixing of a lens plate.
Balancing the Sigma is more difficult than most on the Wimberley WH-200. Although internal, the zooming affects the balance slightly - enough for it to upset the Wimberley. It's not unusable though, by any means. Unexpectedly, the lens comes with a lens cap and an odd choice for a lens with a huge front element. The cap easily dislodges. Although difficult to remove one-handed, I actually prefer the lens covers supplied with the Canon and Nikon lenses (and, it appears, earlier versions of this lens). I've not tried it but the LensCoat Hoodie (Large) looks like it might be a good alternative - the black version is just $13.99.
I have a number of Arca compatible plates but I tend to use the Wimberley P50 with the Wimberley Head Version II (WH-200) head as it's very long and easy to slide back and forth in the clamp when balancing. I also test a lot of different lenses and this fits them all. The P50 is meant for the 600mm lenses but if I add the two screws as nubs it works reasonably well with the Sigma. Wimberley recommend the $52 P20, and I've no doubt that would be the better choice.