For me this is the biggest sticking point. I own a car, and my drive downtown from Bonnie Doon is 8-10 mins. Taking the Valley Line is around 20-25 mins when including walking to the closest station. I like taking the train, and when you factor in parking time, I think this trade-off would be worth it to me. I would rather take the train.
It's the cost that makes this not make sense for me. I own a car, and need to continue to own one. At $2.75/ride with an ARC card each way, it's only going to make sense to take the train if I'm going to be downtown for more than say 3 hours, and only if I'm going downtown by myself. If I'm going with my wife, it would have to be more than 5 or 6 hours. If I'm going with other friends/family, there's not really a good argument for taking the train. I'm never going to take it enough to hit the fare cap, since it's way faster to bike/drive to work than to take transit.
I would love to take transit every time I go downtown, and I probably will more than most would. I just wish that it was cheaper so that it could compete with the cost of driving. Fare zones would help a lot with this, but for now, at the current costs, I'll probably mostly keep using my car.