I had a Bigma for two days.  It's very big, as you might have guessed, and
also quite heavy.  The zoom collar takes a bit of effort as it cranks out
about eight inches of lens barrel in a single turn.  This movement feels
very stiff, and not consistent throughout it's range.  Due to it's speed, or
lack of, it requires fast shutter speeds, and in most cases should be
mounted on a tripod.  It does take very good pictures however.

I ended up taking it back in exchange for a Nikon 80~200mm f/2.8, which on a
Nikon digital ends up being effectively a 120~300mm zoom.  More than enough
for my needs, and much more practical.

Keep in mind the hidden cost, as you will want to hire an assistant to lug
it around for you!  Should you decide to buy the Bigma, exercise caution
when around casual on-lookers.  At first glance they might mistake it for a
surface-to-air missile launcher, and notify Homeland Security!

Bill Crocker

I am considering the purchase of the Sigma 50-500mm lens, AKA Bigma for
nature and sports photography (outdoors).  From what I have read there
are more positive opinions than negative.  What are the experiences of
this group?

Gambo

I bought one in March, use it a lot, even tho it is heavy. I do bird
photography so it works out for me.

Also consider the 170-500mm Sigma, very sharp and smaller.

A friend has one mounted to his Canon 20D.  For convenience it cannot be
beat.  However, the zoom has very poor tactile feeling (a smooth zone, a
tough mid range resistance, then a less than smooth zone).  Image
rendition is average at best.  Not a serious nature lens by any means.
If you have bright conditions and can shoot f/8 - f/16, the results will
be sharp enough for a large print.  f/4 is stingy for 50mm and f/5.6 is
really stingy for the long end, you'll need those bright conditions.

A more favourable review:
http://www.photozone.de/8Reviews/lenses/sigma_50500_463/index.htm