It is, but it works different muscles than what you'd be working outside and on a treadmill. When I do this, I notice I work my calves a lot and get "sore" in those areas. When you run outside or on a treadmill, you have a longer stride because you're not focusing on kicking the furniture!
It's a good workout, but to maximize your training, it's good to run outside when you can (and I know that's getting harder to do as the weather deteriorates).
Find a pace that works for you and go for it. I run indoors on the spot for about 30 minutes and it gives me a good workout.