It depends on how similar the implementation is. If someone clones all or most of another site's design decisions, I'd say that's not cool and is worthy of derision. There are sites that I would consider clones of PadMapper because they try to do things the same way without adding much new or providing a new take on the problem. That says to me that they didn't have much of a vision, and it's annoying - it seems unlikely that if they were re-deriving solutions to those problems that they'd come up with the same answers for everything.
For an example, see how different the results of our both tackling the same problems were - they barely resemble each other in form or functionality.
But yeah, the basic idea of showing real estate on a map is old, I have a research papers from the 80s that talks about making map-based search systems for real estate. housingmaps is the earliest web-based implementation of the idea that I know of, though, and was my inspiration.
