I have a similar style hat from Dorfman Pacific that I've had for about 10 years, and it's fine, but on hot days it's just too much - it's cotton/canvas, but has very little ventilation.  So I saw this and thought I'd give it a try.

So far, it's great.  MUCH MUCH cooler with the mesh on the sides.  The top is a thin nylon-ish material - I'd prefer the same cotton/canvas that the rest of the hat is made from, but it's not a huge issue.

The brim is made of heavyweight canvas - or similar material (I don't really know what to call it, but I think of it as "canvas") - and it's very comfortable.

Issues?  My only gripe - other than the top material - is that the chin strap is sewn in.  I wish it was removable, as I don't really use the chin strap, but I want to make sure I don't need it before I cut it off.

Also, both of my Dorfman Pacific hats are XL, which fit just fine and I have a fairly large head.  With adjustable baseball hats I generally use the last one or two tabs, depending on how long my hair is.  So you can use that as a general size guideline.

Overall - a great hat that I'm very happy with.