Westpark is a a good example, Sam Houston isn't. It's funny you say it alleviates congestion, since the only time there are very many people on it at once is during the frequent accidents at the toll booths.
I used to work off it. I tried it a few times and wound up just taking the frontage road.
There are plenty of people who use the Sam Houston tollway, although it's less than use 610.
If it didn't exist, I'd have to add about 45 minutes to a drive to the airport.
It costs me $2.50 to save 45 minutes. It would probably be an hour if the tollway wasn't in existence and we were all forced to use the existing roads.