Yeah ive been looking at some recently and while they seem good im just not convinced that they are work it, they rance from about $100 to $350 from what ive found. the ones ive been looking have a hot shoe mount and then plug into where the remote shutter cable goes,
I believe that depending on the model you buy as to how quick it reacts. they seem to vary in sensitivity so you can either set it only to shoot when there is bit flases or set for for smaller ones. but i agree with Mat in thnking the reaction time may not be fast enough to get a decent shot, hence why i thought i would try and find someone thats used one, i dont really want to spend the money on one if its not going to do the job.
But on the other hand if it does work well then it would make the task of captureing lightning a whole lots easier.
I will continue to investigate anyway and see if i can come to a conclusion on these things.