I am now an ex user of Merrill Datasite due to their inflexibility to work with you.
They have a decent product (although after doing some further research, there are now one or two players in this market who have better functionality and usability than datasite now) but they are very expensive for what you get, especially when you have other providers giving higher levels of service with better pricing and just as much functionality. Biggest annoyance is you have to use their viewer to do everything which I hate and you can Print Screen things, taking away security of informaiton my clients wanted.
Their interface looks good, but when you dig deeper into the functionality you get lost in the menus and session locks and the nice graphical interface, I just want a point and click solution. Didnt like the way they advised me to do things their way with no flexibility or work around and not listen to the way I wanted things done.
All in all, I think Merrill will enjoy the number one spot for a while longer, but I can see a few newer technology guys fast on their heels and I have switched. I say if you have used Merrills get back into the market and do a clean sweep review, you will be suprised at what is out their and how much they have been ripping off the market like Intralinks used to. The only way is down for them now.