We use Monetate to customize our site based on known personas within the education sector (teachers, administrators, district buyers) and to market our geographically specific workshops to visitors within range of an event.
Setting up segments and controlling several elements on a page is very tricky to manage. If you want to run multiple experiences on a page and then something has to change in the experience it becomes very hard to alter one thing without changing it in a dozen places. I like the built-in WHO targeting but to set up more complicated segments beyond just New/Returning and geo-targeting is really tough.
My biggest complaint is the reporting. I have no idea how many people see which A/B part of an experience vs click on it and then how many of each convert and what their engagement is. It would be nice if the experiences sent onclick data to Analytics.