I use a Sigma 150-500 for almost all of my bird photos. So does SCraig. You're welcome to look at my flickr page for examples; pretty much all the birds in the last year are with that lens.
Mine is great; I love it. But I think the issue is that you can get a really good one, or you can get a not so great one. I got a good lens.
I have no experience at all with the 50-500 and don't even know if it has the same inconsistencies in quality from lens to lens.
However, as much as I like mine, I'm probably going to sell it and buy the new Tamron 150-600 when the Nikon mount is available. You might want to look into that option as well. 100mm more length, as good or better quality than the Sigma 150-500 from what I'm hearing, and a price point of just barely over $1000.
Or (if you're a Nikon user) just wait until that lens comes out and then you can probably get a pretty decent deal on a used Sigma 150-500.

If you're a Canon user, you may already be able to get a deal on a Sigma canon mount lens, since the Tamron canon mount is already out.