250ok is part of our deliverability toolkit along with ReturnPath, some internal proprietary apps, and publicly available tools like Senderscore, Google Postmaster, SNDS, and others. It is very helpful with inbox placement tracking and deliverability issue diagnosis. In our case, it is used across all offices by a dedicated deliverability department.
We use both 250ok and ReturnPath in the company (because we're an ESP) and both have their own pros and cons. Return Path uses panel data while 250ok uses recipients' engagement date for reporting. Both ways have pros and cons but you can't say one is better than the other. They're just different. ReturnPath also offers certification which (I have to be honest) is not as relevant anymore as there is only one way to fix deliverability issues nowadays - by following bulk sender guidelines posted by Gmail, Yahoo, AOL, Microsoft, and other ISPs. ReturnPath has a better documentation website while 250ok could use some refinement. With 250ok you can choose the tools you need and build your own package and save money. With ReturnPath - not so much. 250ok also has partners across the globe while ReturnPath mostly promotes its own services and consultants.
250ok is a great tool for companies that have recently switched ESPs to monitor inbox placement or for companies that historically have been having email deliverability issues and would like to learn about the root cause and improve. Unlike ReturnPath, 250ok doesn't offer certification, but honestly, you don't need certification if you follow the Gmail/Microsoft/AOL/Yahoo bulk sender guidelines. I haven't seen a company which significantly improved their results with that certification and I've been working for one major ESP for 7 years. 250ok is a good diagnostic/monitoring tool but it is not a panacea for those who continue to violate sender guidelines due to poor list/content management.