At my place of work we were a Notes shop for a very long time. From memory we were on 4, 6, 8 and maybe 9. I suppose I had Stockolm syndrome as I didn't really mind it (yes, it was a bit of a shock moving from Outlook back in 1999!). It was someone funny seeing the reaction of new hires when told it was Notes not Outlook. Ironically this also included several CTOs who always said that we'd be replacing Notes with Outlook, but many of them departed without doing so. One potential reason for this was as others have said, Notes was more than just email/calendar. We had various databases and apps in there as well.
But eventually the inevitable happened a few years back and we went to Outlook 365. I think the Windows guys must have done a fair bit of work testing it all as the migration was smooth enough. But it did take a while to tweak Outlook to my needs.
The other side effect, which I find very funny, is when there's a problem with email and some high up bod has a go at Windows, who then point out it's due to something at Microsoft or Proofpoint and there's not much we can do about it!