I'm currently a Mimecast customer having been a MessageLabs customer until 2012.
Back when I signed up the spam filtering and so on with Mimecast was second to none. Now though it's very average. Yes, they've got all sorts of very clever stuff but they've taken their eye off the ball when it comes to stopping spam.
As an example, I'm currently running a PowerShell script against our Office 365 tenant to delete a bulk email containing a malicious excel spreadsheet. Mimecast allowed the file though.
The sandboxing is a great idea but we still get malicious stuff coming through (which Office 365 usually blocks) and each time I raise a ticket with them I get a wolly, meaningless response.
Same goes for the URL protection. It's a great idea but I often see very obviously malicious sites being deemed OK. Again, I get a wolly, meaningless response from support.
They've grown too fast too and for quite a while didn't have enough people in the support teams so I was having to escalate every single ticket because otherwise it'd just sit in the queue for in excess of a week.
It was the market leader (in my opinion) by quite some margin but now, I'm not so sure.