So, has anyone used the new (only in 4.9 build 11) Addon and Parked Domain features of CPanel? How well has it been working? Are the Addon domains basically the same as parking a domain on top of a subdomain from WHM?

not seen it. doesn't seem to be in build 13

whm says its in 4.9 build 11
i haven't tested it yet

just had a play with it, can't say i see the point really

Are the Addon domains basically the same as parking a domain on top of a subdomain from WHM?
Yes, that's exactly what it does. The domain parking works perfectly from the tests I've done.. the subdomain parking doesn't. Once you park a domain on the sub-domain, the sub-domain no longer works.. then when you unpark the domain the sub-domain gets stuck permanently until you manually remove the info from httpd.conf

I actually entered this as a feature request.. nice to see Nick does look at them.. so I added to that the bug report... lets see if he can get this fixed soon. oh yeah, if you also find this bug happens on your server it would be nice if you could addon to the bug report so he knows it is happening to a bunch and not a single incident. ;)
http://web.cpanel.net/bugzilla/show_bug.cgi?id=678