I carry a credit card sized one. In summer it works well. In the winter it mandatory to have the proper tinder. I found that fungi from tree work great. Once they start smoldering they keep going and when you blow gently the ember gets hot enough to ingite cotton or dry grass.
It´ll be a really hard day if I ever need to use it for firestarting as I carry enough other means for that purpose. I carry the lens as magnifying glass. Other poster already mention why.