I set up a Dust Deputy and was disappointed. To see how well it worked, I dumped the contents of my vacuum on the floor and then picked it up using the vacuum with the Dust Deputy.
What I was picking up was a mixture of fine sawdust and some coarser sawdust from a routing operation.
As long as I proceeded VERY slowly, everything went well with everything going to the Dust Deputy and very little to the vacuum. When I went at the normal speed that I usually do with the vacuum alone, more than half ended up in the vacuum (including on the filter in the vacuum).
Based on the above, it appears that the Dust Deputy has some uses, but NOT as a direct replacement for my vacuum.