I have long been skeptical of the Thien Baffle. Though I am wholly ignorant of fluid dynamics, to my eye it seems as likely to keep the dust suspended as to make it drop. Many people are extremely satisfied with it's performance, but I haven't seen anyone actually test it. So I did.
I built setup identical to the one on Thien's website and tested it with and without the baffle in place to see what would happen. (The elbow I had on hand had a large radius, which resulted 4" between the baffle and the top, rather than Thien's 3". I doubt that matters, but wanted to mention it.)
I used a Dyson canister as a shopvac; it has near perfect separation (after six months of use in my shop the filter is still clean) so whatever ends up in the Dyson is what got by the test setup.
For media I used 3 gallons of debris from my JDS cyclone. Since some fines from the JDS had passed through and were lost, I added 3 cups of wheat flour to restore the balance. I built the separator out of the 5 gallon can that came with my Oneida Dust Deputy.
I sucked up the 3 gallons with the baffle in place and got 8 ounces of dust in the Dyson; about a 98% separation. I then removed the baffle and tried again, after mixing the 8 ounces back into the debris.
Without the baffle I got 3 ounces of dust in the Dyson. Repeating, I got 5 ounces.
Putting the baffle back in place, I got 11 ounces.
On the basis of these test it appear that both separated out essentially all but the finest debris. It would take more than 4 runs to determine which was better with the fine dust, but it is fair to say that the baffle was not significantly superior to no baffle.
I only got the bin half full (it was all the debris I had in my JDS) so I cannot comment on claims that the baffle is much better when the bin is nearly full.
I used a feed rate comparable to the one on Thien's website. Normal shopvac use would be much slower. I have no idea how it would affect the results; I simply didn't feel like spending a few hours feeding it slowly.
Most people seem to be using them on DCs. I don't know how meaningful my results would be to how it works on a DC.
I didn't test the Oneida Dust Deputy, but I have emptied it twice of mostly sanding dust since I got the Dyson, and less than an ounce wound up in the Dyson, so it is pretty good. And after all this the Dyson filter was still clean!