I've used Ruby as a sysadmin tool a lot--actually, it replaced Perl (sniff) for me in that role. So I think it's generally pretty useful. But I can't speak to its application to, say, writing games, or applications using an OS's windowing systems, or scientific applications--all areas where I believe Python has a lot more traction. I know you can do that stuff with Ruby, but I don't know how it compares, as far as maturity of the libraries in those areas.