The gill mask is almost perfect thanks to its reusable design. One issue I've had when inserting a new filter is that I get little gaps in the corners, on one side, that aren't covered by the filter and is therefore unfiltered. This can be somewhat adjusted for by pulling on the filter while inserting it.
I also find it hard to know if I'm performing the seal check correctly, as there always seems to be escaping through or around my hands.