I often ride in areas filled with thorns. Using regular MTB tires, I'd come home every other time with a flat. Now I'm using slime filled tubes and I never change a tube in the field - although sometimes I have to fill them in order to get home. Once a month or two I have to change one. I'm thinking of trying Kevlar tires but my budget is limited. I've seen Bell Kevlars for under $15. Has anyone got any long term experience with these tires? How do they ride and more important how do they hold up to thorns compared to the more expensive ones? I've also seen Bell slime filled tubes for as little as $3. Are the more expensive tubes any better?