What do you like best?
It is integrated and optimised for mail (and not for calendars, tasks and other purposes). This keeps the software fairly minimalistic. There is support for Exchange, GMail, IMAP, POP etc... In fact, I never directly read my GMail in the browser.
While my colleagues work on Windows and prefer Outlook, I usually have no problems collaborating on my Mac. And I have the feeling that it is less heavy than Outlook, which tries to be everything and even more.
I especially like the lighting fast searching throughout all my mailboxes (thanks to spotlight indexing).
What do you dislike?
The account management is shared between the os and the software and your iCloud account, which may occasionally re-create accounts you don't need anymore.
Syncing read status between my iPhone and my laptop is not fully working with the way my company's mail is setup, but works for other mail providers.
I've had a few cases where my mail blocked, due to corrupted mboxes. This could take some tinkering. Loading off old mails to a local or archive mailbox may help things a bit. But luckily this only happened a few times.
Recommendations to others considering the product
If you are allowed in your company to use it, I can recommend it on macOS. For most mail providers, it works just fine.
What business problems are you solving with the product? What benefits have you realized?
It is my main (and only) mail reader on macOS. The only time I use Outlook are on Windows or when I need to do a mail-merge from a Word document.