I have a CREEE based lamp. I think I've only had one lamp fail due to a power cord coming loose. Oh, and I had one get stolen (and moved the rear taillight to the bars and went with visibility only lighting).

I don't carry a spare lamp, but I do try to carry a spare battery pack, but I think my current battery pack lasts several times the length of my average ride. Waterproof battery packs are handy, or at least putting the battery pack in a plastic bag.