I'm a BIG fan of Gill masks. They provide superior protection and comfort, but they are best in calm and climate controlled situations.
The fit and seal are fine. Neither of my kids or I (in a large) have had any trouble breathing in them.
It will get swampy if you exert yourself or wear it in warmer weather.
Deducted 1 star because the filter door can pop open if subjected to jarring activity (like when my 8 year old jumped off a 4 foot high platform on the first day of school). A little strip of clear packing tape will keep it closed, but I feel like that shouldn't be necessary.
The Gill masks work GREAT for calm activities (sitting on train, walking in store, going to class), but you might want to have something else handy for gym and recess