The choice of a focal length is really very personal. I came to the 24 from the 21, a focal length I didn't like after working with it for about two years. I immediately loved the 24 and use it quite a bit. I think the thing that bothered me about the 21 was that you have to fill up the foreground so you need to go looking for subject matter that suits the lens (perhaps the wider you go the more true this is). The 24 is a lot more suited to happenstance as you're wandering around.
If you want to try the lens there are plenty to fit a RF. CV has two, the LTM and the M-mount, Zeiss makes the 25, and Leica makes three 24mm lenses. I use the 24/2.8 Elmarit and I'm thinking seriously about getting the new 24mm Summilux. I want to see more pictures first though. My main use of the 24 is as a third lens when I'm traveling. My travel kit is a 35 and a 75 and the 24 is there for scenes that are a bit too wide for the 35.
If you want to see some really excellent pictures using the focal length take a look at the book Invasion 68: Prague by Josef Koudelka, just about all of those pictures were taken with a 25mm CZJ Flektogon. Below are a few shots from my Elmarit taken on a recent trip to Berlin: