My Buff Filter Tubes arrived today! Honestly, if you've worn a buff before, you know pretty much exactly what to expect. The sole differences are that there's a hexagonal filter pocket sewn to the interior front-top of the tube, which is large enough to cover my (substantial) nose and mouth, and that the top of the tube is folded over and hemmed, as compared to the raw edges typical of most buffs (as well as the edge on the other end of the tube). I do suspect that the hem might make for a snugger and more consistent fit over the nose and around the cheeks.
The filter pocket on its own doesn't affect breathability, as it's a loose mesh, and I don't want to burn a filter testing things out (they only come with five disposable single-day filters each, although you can order packets of 30 replacements), so I haven't tried one yet. The tube comes with an informational booklet about filter performance and guidance on wear, and states that the filter meets Type Ia and Type II classifications for bacterial filtration and breathability. Based on other products I can find with those classifications, I expect breathability and filtration performance to both be similar to a standard surgical mask.
While regular buffs hold up to machine washing/drying fine, I do suspect that the filter pocket on these might be a bit more delicate, so I'll probably machine wash and hang to dry.