In Dallas TX our lightrail runs about every 15 min too. It's works pretty well and is big enough to be actually useful. I like to use for big events when you know parking is going to be a nightmare or expensive. It's slower than a car though, the trip about doubles in terms of time taking the train vs driving.
On a lot of London Underground lines, 5 minutes is considered poor service in rush hour. Every 2 minutes is required to handle the sheer volume of passengers