I have had the same Photon II on my keychain for going on 15 years and besides one battery change, I have not had to worry about it. It works every time I press the switch and it has sat in my sweaty front pocket through many a hot, sticky East Coast summer during those 15 years. (I just checked it--yup, lights up!)

I can't comment on other brands, but I would stay away from any super cheap ones unless you keep two or three handy. So, if you get one as a freebie or it costs a buck or two, I wouldn't count on it as an EDC light. That said, the cheap ones are great to keep a stash on hand to give out during a blackout and not worry about getting them back.