I started wearing these several years back, about all I wear now. They come in several styles, colors and materials, so make sure to find the one that suits you.
Turned my buddy onto them and he uses them as part of his uniform (carpet cleaning, better than slacks). I prefer the ripstop stuff and the nylon if you can find it. Most of them accept foam pads in the knees like military uniforms do. Which are also good.
Edit: like I said, they come in a lot of different styles that include roomy tactical to tighter fitting "slacks" that have the side pockets but aren't so tactical looking. Different waist bands, belt loops and pocket configurations. Just a few of the materials are cotton, cotton blends, riptstop, nylon and firehose material (very stiff, very heavy duty). They have expanding waists and those come in various configurations too, but ultimately it boils down to them being very comfortable even with IWB carry, which they had in mind when making these. Some even have a pistol mag pocket on the front left thigh and metal rings for tethers or tie downs.
I guess if you're in the army or need something like that or just have the money, the higher end Crye stuff is good.
Tactical Concealment makes a pair of awesome ghillie pants that have a padded waist, nylon cordura skid plate in front, bar tacked 550 cord grid on rear, large pockets on side/back of thigh/ass easy to access in prone, and other features, just very well thought out BUT I understand doing business with the guy is like pulling teeth and he's had a lot of complaints over the years, don't know about now. If I was still in the army I'd probably be very interested in some of his gear. Hell, I'd like that Viper tog now anyway!