I've used Terapeak for a couple of years and have really liked the information.
It used to be well laid out and had a user interface that crushed HammerTap's garbage UI.
In the spring of 2008, however, Terapeak rolled out a new user interface that disabled a number of features and frankly skewed the data bad enough that I use them only for limited, focused research instead of broad, browsing research.
Examples include the Categories graph/chart has lost it's links to drill-down into deeper data the Categories left-menu tree now shows sub-categories with a Total Listings value rather than the Total Sold value the sort-by End Price is often inaccurate, resulting in a jumble of non-sorted results and the premium Hot Research is limited and lacking punch.
I do like the new Date Range selector and the higher Saved Search Query limitation.
Support is clearly overloaded, with the development team able to resolve serious issues in terms of months, but at least they seem to be trying.
Overall, I still prefer Terapeak (certainly over the over-rated HammerTap service), but disappointed in the design changes introduced in early 2008.