Tough doesn't cover it.
This is a seriously long term review on an old set of pants. I got these for a steal on SAC back in the summer of 2007. Since then, they've been my go-to pants for everything from camping to backcountry and frontcountry skiing.
The Thetas are seriously tough. Mine are patched on the left thigh, but it took an encounter with a rock that cut all the way down to my leg to slice the pants. Other snags, tree branches, etc. haven't even left a mark.
Features on these pants are great. The small pockets on the bibs are perfect for holding a ski pass or snack. Almost full-length zips make for easy on and off, even over boots. The gaiters keep the snow out, as expected. The fly includes a second slide that zips up from the bottom, allowing quick bathroom breaks without stripping down to your skivvies.
Fit-wise, these have stayed with me from my journey up to a 36" waist and back down to 34". The length is good, coming down over the tops of ski boots on my 31" inseam without looking like a disco star. I expect to be wearing these pants for a long time.