I'm trying to select a scope and have been looking at a 6.5 x 20 x 50mm Sightron Big Sky. Anybody use one of these and if so do you like it?

I doubt many people are using it because the 50mm objective will add unnecessary weight to the rifle. I say unnecessary weight because silhouette shooting is always during daylight hours when the light gathering ability of the 50 is not neede. I suggest getting their 6-24 SIL model instead. It has great glass and 24x.

Get the 40mm silhouette special 6x24 from Chicken George. His link is at the top of the page. Great scopes, I have two of them.

and if you shoot indoors at all, the 6-24x42 SIL focuses to 50' no problem.

I purchased my SIL model from George last December and have been using indoor 50 foot and had it outdoors in January for our Frosty Iron Match in 22F Temps. There is a topic " Scopes and Rings" and you will find detailed info on the Sightron SIL model as well as others.

If I read the specs correctly the 50mm has side focusing. Is this a feature I would need or not?

Nope, not needed.

My previous scope was side focus and sold it to another club member last month. I am left handed and found it awkward and did not have enough distances marked for quick adjustment. I used white out to mark the distances which worked. There was hardly enough room for tape marking the distances on the side focus Vortex Viper scope. Perhaps is not an issue with others.