I like that it has an easy to use interface for all of my emails at one place! It's so much easier than managing them in multiple places now - which was what we were doing before this service came along!! Nothing really stands out as being bad about smx though; everything seems pretty good overall, but you can't go wrong with any product from Smex if your looking into webmail services! Having our own private domain means less work when sending/receiving mail internally or externally via other providers (like Gmail).
We've been able solve many problems related to spam filters by using their products such as SpamTitan Pro & Cloudmark Email Security Platforms together within MailChimp accounts across different domains.