I did an extreme search on this lens before buying it. I wanted to go into surf shooting and needed a long piece of glass. I've started to check out prices for 500mm and 600mm primes and the truth is i could not afford none of the good lenses (Sigma 500mm F8 was not an option).
Then i've heard about the "The sharpest extremely long-focal-length zoom we have ever tested, and incredibly good short-focal-length performance...." - Popular Photography review and start to look at some pictures.
This lens is known in Dpreview.com Canon SLR Lens Forum as the BIGMA, it's reputation is on high standards and the samples were very good, only one thing was missing, surf shots !
After another search on the Nikon SLR Lens Forum, i've found this website - Unrealimage.com - in wich you can see some surf shots, all of that shots were taken using the BIGMA. Altough the samples were small, i e-mailed the website's admin to send me a full sized image. It was a proof, BIGMA could offer me good color contrasty, sharp and clean surf images.
It was a no brainer, i saw the lens at 1100 € and could not hesitate, i bought it !
After a 6 month trial on a Canon EOS 10D, i think i'm prepared to describe my feelings about this big piece of glass:
Construction - It feels good holding it, the rubber rings for zoom and focus are quite nice. The tripod/monopod collar is very versatile and it's easy to go to "portrait mode" when you need to. It's a little bit heavy, but you won't get lighter at this focal range. The 86mm filter size is a negative aspect, since a single skylight filter can cost up to 65 €, a polarizer won't be needed tough.
Tip: Always lock the lens when carrying it, the pump zoom goes down down when moving and can seriously damage cap/front element.
Mechanics - The AF (HSM - HyperSonic Motor) is very quick, silent and accurate... well sometimes. I must say that when in AI Servo i take a sequence of shots (burst mode) and then pull my finger out of the trigger, and immediatly after that i press it again to go for another shot, the focus hunts all the way to 1 feet and then to infinity. I've missed a few shots because of that, and it is very annoying when it happens but, as i said before, it's not all the time.
The pump zoom is ok, remark that the lens almost duplicate it's size when in 500mm.
Tip: I've seen dozens of sucessful handheld shots, but i always shoot on a monopod or a tripod, it really helps to avoid camera shake/blurry images.
Image quality - My working distance is between 200-500mm but i think bellow 200mm it's almost the same. I must say that BIGMA is very sharp troughout the entire focal distance but at 500mm, it looses a bit of it. I really don't care about the apperture of this lens, some guys say that it's slow ... but i can get some very very shutter speeds at lower ISO. For example, on a overcast, non foggy sky my settings are like: ISO 200, F8 and 1/1250s - 1/1500s. On a clear sky i'd get: ISO 200, F8 and 1/2000s - 1/4000s.
I shoot majorly in Manual mode (except when random clouds appear) and i always shoot surf at F8, it's the lens sharpest apperture (F9.5 and F11 are also very sharp). It delivers very fine results, colors and contrast are at high stakes.
TIP: It's funny to play around with portraits since it really isolates the individual with background blur.
Conclusion - The 50-500mm f4-6.3 EX APO RF HSM is an excellent lens for a certain kind of photography. It's very good for wildlife (when good light conditions) and for outdoor sports like surf, football, soccer etc... Well constructed, it offers good colors and contrast, being sharp trough the entire focal range, providing nice, clean pictures.
One thing i must say is that every single picture that came out of my EOS 10D went to the digital darkroom for some adjustments. I also have a Canon 16-35mm 2.8L and the images taken with that lens need to go trough "tunning". So don't expect that a picture with the BIGMA or any other lens would go for a magazine cover without post processing.
It's image quality didn't left me down one single time, and i've been working with several newspapers/magazines. I would even consider it a professional lens for a medium market , but as said before, for a certain kind of photography. It paid it's cost twice (in 6 months) selling pics to surf magazines and newspapers, and will continue to do it for the next couple of years.
I think that's all, i did my best, anything you want to ask, a critique you want to do, please, send a email.
Thanks for reading, and sorry for my poor english.