I weat Firstgear MeshTex jacket and pants. While not like wearing nothing, it is hands down much more comfortable than non-mesh. Wore it last summer on the TWAT (Trans Wisconsin Adventure Trail... Get you mind out of the gutter!) with temps in the humid 90*'s and was much more comfortable than a friend wearing a MX armor vest and jersey.
One thing to keep in mind when wearing mesh is that you dehydrate faster due to the wind blowing past you skin. Hydrate or die my friends!
01 Jeep XJ w/ 200,000 miles as of 3-26-11