I have used the light rail many times. It's my favorite mode to get to the game. I usually catch the rail at the Museum of Fine Arts Houston (MFAH) and park in their parking garage (they have security). When I started it was cheap, $5, but now they charge about the same as the stadium does ($15). You pay in the lobby at the desk. From MFAH the train takes 25-30 mins and it's about an 8-12 min wait between trains. The trains aren't too crowded going to the game so it''s nice on the way down.
On the way back, it's a different story. You have to wait in line in the hot sun to catch the train back. Be prepared to avoid sunburn. You may have to wait 30 minutes or so to catch a train. The trains are uber-crowded coming back. YOu'll probably have to stand and you'll be packed in most of your ride back.
If you use the MFAH for your parking, you won't have bad traffic once you get back to your car