I use dashboard all the time, especially the unit & currency conversions, world time, addres book, calculator, eBay watcher, weather and TV guide (worldTV). Others I regularly use are Australian whitepages, yellowpages and postcode searches, CPU temperature monitor and language translation. I also like to keep a few references to hand, such as a table of the elements, Wikipedia, etc.
Once you find what you actually need and don't need (and believe me, theres a lot of Dashboard stuff that you simply don't need) then Dashboard is incredibly useful.