If you are talking about a light to use up your batteries, the U85 will indeed allow you to run for a long time even with pretty empty batteries.
But.... I think you'll quickly find using the U85 to use up old batteries is like buying a Ferrari to drive two blocks to the bus stop.
Also, you'll find that you are not going to use up the batteries as fast as you produce them with the M3. Especially because the U85 is VERY good at this... you'll get tired of running at low output long before it has extracted all the power out of the cell.
I highly recommend the U85 (or the B60 or U60), partially because they can run for a long time.
However, I haven't heard a winning strategy for "using up" old batteries yet.
-john