I recently sent my Tamron off to get the VC firmware update as mentioned in the Tamron service advisory performed. $50 USD shipping for the pleasure of a free fix. They performed the needed update as well as a cleaning and "adjusting focus issues" (their words) and shipped it back to me last Thursday. Right after that they posted yet another Service Bulletin that the firmware needs to be updated so it will work with the 5Ds & 5DsR cameras.

Did they put in the most recent firmware on my camera? Or do I get to send it back yet again when I purchase the 5Ds?

This is one of the best reasons I can think of to go with the Sigma lenses, even if they were dead equal otherwise (which they seem to be). The value of being able to update the firmware on your desktop is huge. Plus, you're not without the lens for weeks on end. While Tamron's response and turnaround has been exemplary in their customer service department, my issue lies elsewhere with the company.

If I were serious about using this focal length, I'd probably consider the Canon EF 100-400 II and the TC as Kayaker72 mentions - because the results I've seen are excellent. Plus, you've got a much more manageable (and insanely sharp) lens when you don't need 600mm. I think for the added price, the performance and peace of mind will make up for that pretty quickly!

I'm interested to see just how it performs as I didn't mention anything about focus issues. As far as I knew it was fine! The dust they found (front element) was incredibly minor, it took a flashlight to see it! I really only expected them to update the firmware - and am just a little bit anxious about them taking it apart. I'm kind of a "if it ain't broke don't fix it!" type. I'll report back if I see any improvement with the lens above and beyond being able to leave the VC on while in panning mode. Or, if something is worse!