Ease of use, especially since the new user interface. I've been using a number of years both at my current employment and my previous. Support is also very good. UK based, Dan and Ian will always be able to help. In terms of the product, once setup, its as easy as adding a user to a group, they will then get the signature on every email. rules also allow you to have more granular control, if you have a campaign running for a few week as an example, you can setup a rule with dates to apply the signature block, then say external only, for people in this group. Fantastic product.

It is easy to have different areas have slightly different signatures

the fact that i can use it across multiple devices without having to store onto the device itself and that I can update it at my leisure