Document portal

I've become quite interested by the whole concept of document portals lately.  You know the ones - you upload a client's tax return for example, they log in to 'their' portal space, view the document and then are somehow able to digitally sign it.  With some of my clients zipping out and about over Europe and beyond it seems quite handy.

I'm aware that there are a few such portals about, so was wondering if anyone around here has been using one of them.  Would you recommend it?  How has it worked out for your practice?  What are the costs involved, and what different providers are there?

Thanks,

WS.