This Sigma long telephoto zoom is a cut above your average 70-300 level of performance and construction. However, it doesn’t have that type of lens’s usual advantage of portability - It’s a big heavy lens at 1245g, but it feels very well made with smoothly operating controls with just about the right amount of resistance to the zooming action, and it’s finished in my favourite Zen rubberised coating. Performance is excellent at the shorter end of the zoom range up to about 300mm, but results are still perfectly acceptable at 400mm and full aperture.
I wasn’t too surprised to find that my old Sigma 400mm f5.6 APO is slightly better at full aperture, but the flexibility of the zoom means that the Sigma 135-400mm gets used much more. Certainly the performance of this lens at the long end is much better than a Sigma 70-300 APO Macro II I owned briefly. That lens was truly excellent at 70-135 but faded quickly to not very good at 300mm. So if you’re wondering if this is a better lens than the Sigma APO 70-300 and most other 70-300’s, the answer is yes, quite a bit better. You might need a bigger camera bag though.