If you end up deciding you can live without the stabilization, another lens that might be worthy of consideration is the Tamron 200-500. It's smaller and lighter than the 50-500, hand-holdable but definitely not a casual walkaround lens. It's capable of razor-sharp results, and will operate well in decent light. Focus has been fast and accurate with my Alpha body, and the lens is surprisingly sharp even at full 500mm and wide open. It's my primary birding/wildlife lens, and I walk miles every weekend with it. The good thing is that it's quite a bit cheaper than other lenses with similar range - usually running in the $850 range new. I've got lots of samples in my galleries with it...this entire gallery was shot with this lens exclusively:
http://www.pbase.com/zackiedawg/a300wakodahatchee
Downsides? Though lighter and smaller than some similar competitors, it's still a fairly big lens, so it isn't going to be a Disney walk-around style lens. Use it for wildlife, whale watching, birding, etc, and it's in its element. Also, it doesn't have any stabilization...don't know how important that is - some seem to cope just fine without it - and in good light, I don't see it being too problematic. But in lower light, or unsteady hands, it might not fare as well. My Alpha body has in-camera stabilization, so I might be reaping some benefit from that, making it hard for me to say for sure if you need it.