I like that it integrates with MailChimp so easily, as well has all of our other marketing platforms! That's awesome for us because we have several sites/marketing campaigns etc across different providers which makes managing them much easier than having multiple accounts in each system separately (which would be far too complicated!).
The support team can sometimes seem overworked but they're always quick at responding when contacted via chat or ticket - their responsiveness really helps make things easy during busy periods especially if there isn't anyone else around who knows how these systems work internally from experience!! Not many downsides yet though nothing very major either ;) We've used this product since 2013 mainly just adding contacts within various emails through its API service where needed however recently started using again now due to an increase in traffic levels after switching my hosting provider out last year & needing more storage space.