Veterinarian with 12 years experience here, currently working for Banfield for the third time. (Yes, third. LOL. Left once for a private practice, which had psycho owners. Came back, then left for a different corporation that had really crappy medicine. Came back again. Now work part-time and not planning to leave again unless it's to leave practice completely.)
Pros - pay, benefits, good quality medicine (maybe not "amazing" compared to some high-quality hospitals, but definitely above-average for my geographic area)
Cons - emphasis on metrics/production (though I've honestly experienced that in private practice too), dealing with clients who are frustrated by wellness plans and not understanding what's covered, emphasis on wellness care means you don't necessarily get a lot of emergency/surgery/etc. experience
I've worked at several Banfield locations and, as said above, there is a LOT of variation between hospitals as far as management, culture, etc. I like the fact that there are certain minimal standards that must always be met, though. You'll never be expected to anesthetize a pet without pre-anesthetic bloodwork or an IV catheter, never be expected to try a surgery you aren't comfortable with, never be pressure to do surgery under minimal sedation, never have to perform tail docks or dewclaw removals, etc. I've worked in some pretty sketchy practices, so the standards at Banfield are a huge plus to me.
Also, someone made a comment above about not having any flexibility in medical protocols - that's no longer the case. When I first worked with Banfield (2009-2012) they had just started phasing out the whole required-protocol thing. Now it's completely gone. You can practice as you see fit, as long as you're meeting their standards (which are really more of a baseline than anything).
I'd be glad to answer any other questions you have. No job is perfect, but Banfield is definitely the best fit for me at this time in my life.