I have seen kamen rider black and RX. well i have seen all the series out on HK dvd's.
If their was ever a series to have in your collection, kamen rider black is defiently a series to have. If you don't have it, then you are missing out on one of the greatest series ever.
It is a pretty dark series, but one of the best rider series i have seen. don't get me wrong i have enjoyed V3, black rx, and all the new rider series.
But with Black, you would get so into the story. that you end up watching it one after another and seen it within a week.
here is the link to japan hero about the story of the series -
http://www.japanhero.com/kamen rider reviews/kr_black.htm
Edit: Ooops just notice you was after black RX, well unless you have seen black first. black RX is only good as a sequel, it is a poor series and does not live up to its predecessor.
It is still enjoyable, but to a point. its too light for a rider series.
Only get the series if you are looking at watching all the rider series and want to add it to your collection.
any else, then its only good to watch over time. not something you can seriously get into. the only good bit of the series is seeing kotoro again.