Interesting comparison by Tyson Robichaud:

I think the 15mm is more a replacement for a lens like the 14mm or 17mm’s of the system, but seeing as I also have the Pana-Leica 25mm lens, I can more easily justify passing the 20mm along as well. I’ve already sold and reacquired my older, original 20mm, which I then sold again after getting the newer version 2, so I do have some type of connection to that lens as I feel it has been one of the best balances of size, speed and quality for the system since it’s original release all those years ago. That said, I think this new 15mm, coupled with my ownership of that 25mm lens, will not allow the 20mm to get the exercise it deserves, but that isn’t to say that we couldn’t also be having a conversation that for a more budget minded shooter, the 20mm could be an ample replacement for both the PL 15mm f/1.7 and PL 25mm f/1.4, so having to deal with these types of quandaries in a system is pretty awesome.

If you own several lenses in a similar focal length, streamlining your kit is always a good thing to consider.