thanks alot for the replies gents
they caught my eye browsing on eBay and did some googling the other day, some people think it makes the tone brighter, some think it makes it warmer. some people say it affects the open strings only (makes sense to me) and some say it adds 'something' to all the strings.
if it would warm up a bright strat a bit, adding sustain and looking cool in a retro 1980s way, then i would be tempted to have a go with one.
i have a Gordon Smith with a brass nut fitted in fact and i dont mind the patina it gets, but i've never done an A-B test with and without the brass.
gathering all the information i suppose, overall, that the effect of it is quite a subtle one