We are running SM Enterprise Unlimited Mailboxes on a virtual server with a 2.10 GHz 6-core Xeon Silver CPU, and 8GB RAM running Windows Server 2012 R2. We've got 524 domains and several thousand users.
I believe we're now on our 3rd Beta version of SmarterMail, after installing around Christmas (which wasn't the first Beta version out - probably around the 3rd version marked as production ready). And yes, we are running this in production.
We've yet to witness any mission-critical errors. We haven't officially notified our client base that MAPI is an option, but we run our own email on our mail server and I've tried MAPI on my own accounts - other than an issue with attachments which has been acknowledged and I'm sure will be fixed sooner than later (and doesn't appear to be a widespread issue), things are working pretty well so far using the basic features. We will be letting a handful of our customers know that MAPI is now an option and letting them test it out this week as well.
As far as other protocols, EAS has done some odd things for me personally both in Outlook (before I converted to MAPI) and in mobile devices in that it sometimes will not sync non-email things (like contacts and calendar items) in both iOS and Android. I don't know how widespread those EAS issues are but it appears there may be others in our world seeing strange things. We don't have more than 30 of our thousands of users using EAS so this isn't a huge concern for us at the moment - and email syncing on EAS appears to be fine.
POP / IMAP are likely to be working just fine since nearly all our users are connecting via those protocols and we've yet to hear about any issues.
Very few people in our world use EWS and those who are out there haven't reported anything so I'd have to guess that's working normally too.
So, for everyone YMMV, but for us given the limited EAS exposure we're pretty pleased with the current production-ready Beta in all other protocols including MAPI. The EAS issues we've faced aren't without viable work-arounds and may have some other fixes too, but for anyone with lots of EAS users on mobile I would proceed with caution.
As far as performance, we've yet to witness any significant improvements but there's also been no degradation. Things were running a bit sluggish before I posted this and it appeared to be quite high RAM utilization on the server - possibly being caused by ClamAV. After restarting the SmarterMail service things seem to be back up to speed but that's the first time I've had to do that since running the Beta.
I'd be curious to hear about anyone else who has been running in production.