I saw a level 8 gym stacked with the most powerful group of pokemon I ever saw, I think all around or above 1500, including a 1800+ Snorlax. They still got toppled within a couple of hours. If you're in a busy area it seems completely pointless trying to get a gym.
It's pretty pointless trying to HOLD a gym, but getting one for your team still gives you a good chunk of exp and gives you the daily reward. I like the system, it makes it most rewarding if a gym constantly changes hands.