I like how easily you can set up groups with multiple people in them for different purposes (e-mail distribution lists).
It's easy enough so non techies don't need any training or help setting it all up themselves! We have been able use this product very successfully since we started using scalixir about 2 years ago - there is no downside at present time as far as i am aware. They do offer some support but not much more than what would be expected from an enterprise level solution provider such as Microsoft Exchange Server etc.. Nothing really comes into my mind right now when asked by our clients if they are happy/dissatisfied overall?