Basic facts: Rapid Rise (aka - low normal*) derailleurs have been out of production for at least 8 years, maybe more, so these things are rare and hard to come by. They were hard to find even when they were in production, about two years The cool bike packing / touring-in-the-middle-of-nowhere benefit of rapid rise is that if your derailleur cable breaks, the derailleur will go to the lowest gear on your cassette instead of the highest (like a regular high normal derailleur will do). That makes it easier to continue your ride until you have a chance to replace the cable. This is especially important if where you're at is really hilly and/or you gots lots of baggage on your bike. And you pull both the front and rear friction shifters in the same direction when shifting to the high or low gear so that's a small bonus for those who appreciate logic.
For 9 speed indexed or friction shifting drivetrains.
Opinion de Grant: All rear derailers should be Rapid Rise. It means you move the left and right shifters the same direction for the same effect, instead of right-forward to get higher in back and left-back to get higher in front. I thought it was goofy when Shimano came out with them--and I think it was more like 12 years ago, but whatever. I was a naysayer until I used one.
They didn't take off because racers didn't dig them, and the average rider and conservative product managers at the big companies didn't want to risk turning off conservatives and worriers. I was one of those, so I know.
It seems like having one bike shift one way and another another would be confusing, but it's so tiny, inconsequential. You mis-shift now and then, and just correct it.
Forty-eight dollars buys you the best rear downshifting (to lower gears) you'll ever experience. Shimano knows this is better. When they gave it up they must have thought, "OK, dumb world." The engineer who believed in it and the guy who pushed it through must have been incredulous.
You have more than one bike? Try this. It's not worse, it's different. It won't make your life different, but you'll probably like it. The most dramatic improvement is when it's kind of too late to shift to the big rear cog as you're heading up a hill. These make it much easier and better. They will NEVER be made again. I am philosophically opposed to dumbing down a bike too much, but I'll take this much. I like that it's counter-status quo and clearly better.
I don't have a RR derailer on all my bikes, but everytime I ride with one I think I should. If I had Michael Bloomberg's money I'd use 1/2,048th of it to pay Shimano to make a RR Altus with the quality of a '92 XTR. I'd want enough to give to all of you and family and friends.
Can you afford a $48 experiment? If so, go for it. Then you won't have to listen to any opinions (mine included). See what it's like. This opportunity won't come again, and it's less than half a hundred dollars.
Note: These are unboxed and they have scratches and a little dirt and dust and grease on them. If that bothers you then please do not buy. They are functionally perfect, and if they were pristine in a box, they'd be dirtier after a ten minute trail ride on hardpack than they are now. They are New Old Stock (NOS), i.e. they've never been used, never been installed on a bike. They're not NIB (new in box). Hangar rash speciales!
For wide range gearing—like for touring or trail riding.
Shifts to 36t. An excellent choice, always. Nobody,ever squawks about a Deore.