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I went with David to Botswana last year with the Bigma OS (not the Bigmos = 150-500 OS). Be aware that there are a lot of different permutations of Sigma big lenses.
Out of all of them I like the Bigma OS the most, although I bought the 120-300 OS and I think it is really good as well. The 120-300 OS performed very well for me in Antarctica last year and that was comparing it to the new 400 IS 2.8 II and 70-200 IS 2.8 II. The only reason that I didn't bring the Bigma OS was that it was still under repair from Sigma Canada (12 weeks and they didn't get it done the first time :P)
I have the 150-500 OS (the lens you are asking about), 50-500, 50-500 OS and 120-300 OS. I also have the 100-400L (2008). I'd rank them wrt to optical quality as 120-300 OS first followed by 50-500 OS, 100-400L, 50-500 then the 150-500 OS as last.
For me, the Bigma OS wins as most versatile while providing very good optical quality. My only gripe with the Bigma OS was that it got pitched out of a moving vehicle on the first day of my Safari to Botswana last year with David. It shot semi OK but was decentered on the left on certain shots, and was repaired by Sigma (replacing internal elements that were chipped )
Given the choice between the 150-500 OS and the 50-500 OS, I'd pick the 50-500 OS.