I've only had mine in for a short time but I have two riding buddies who have put thousands of miles on them without flats. One is riding Michelin Krylion and the other Schwalbe Durano, which are both pretty tough tires and I'm sure some BF members have done similar mileage without flats and without liners.

However at only 30g per liner they seemed prudent to me. I'm still carrying all my spares though-- no liner will stop a roofing nail, broken glass, etc. (The key to those is obviously seeing and avoiding.)