I've used it for four years and like it.  Clients are split on it.  Most love it, but some struggle with it.  I have my admin work with them on it as needed.

Watch the videos / read the guides on their site when you start (so don't wait to implement until tax season).  It will save you a lot of start-up pain.

Easiest way to import your clients is to 1) make sure email addresses are good in screen 1, and 2) print organizers (or tax returns) to SmartVault.  It will create all your clients, and you can invite them with the click of a button.