I have the corded version and use it regularly. For trucks, i find it very useful in cutting away bolts or other parts that have seized up, and like others have said, and angle grinder just is too big. Those cutting wheels will cut through anything, plastic, wood, steel, you name it. I also like the grinding bits if I need to smooth something down a bit, or make it a bit smaller to fit correctly.
I definitley recommend the corded version, and also variable speed. High speeds can burn or melt certain materials, so being able to throttle back helps get a good cut.
I keep scrap steel from old computer cases and with the dremel I can fabricate small brackets and other metal pieces for projects I am working on.
There is also a spiral cutting bit that will allow you to cut out openings in panels like a router or maybe a jigsaw would do.
All in all its handy if you are doing any kind of small or precision work that other tools are too big or inacuurate to handle.