Wow. The stairs on this trail really are a killer. This was one of the most difficult hikes I've been on in the central Texas region, and it's all of nine miles from my house! I had planned to go the entire length of the trail (starting from the pond at the South end), but I had to turn back about three quarters of a mile early when it became evident that I was going to have trouble making it back to the car if I kept going. I'm actually going to have to work up towards completing this trail in its entirety.
Strenuousness aside, this is really a gem of a trail. It's well marked, extremely well maintained, is lightly traveled (only two groups passed me while I stopped to eat lunch), very scenic, has a variety of wildlife, and is nonetheless smack in the middle of Austin. You can't ask for more on an urban area trail, I don't think.
Do note that numerous areas of this trail are not wide enough to really accomodate two people. Be very careful when passing groups going the other direction, and find a spot to step aside off the trail where possible.
I'll have to try this trail again after we've had some rains. The photos posted here from when the streams were actually running make the area look even nicer.