As a newbie gun owner, here's been my experience...
The first time I've been to Magnum was about 5 years ago... not gun related but because someone I know had some business with Art. My friend introduced me and Art just kind of shrugged and didn't say anything. That pretty much is my impression of Art since I've had no interaction with him (and I have no desire to). That said, I've been to Magnum recently to try and visit all gun stores in Hawaii and have found Brian there to be knowledgable. I've also gone there with a friend to use the indoor gun range. I hope that the issues going on with Art will help the store change for the better.
I like the folks at Young Guns. The place is very convenient for me as I live closeby. The folks are pretty friendly and I like the relaxed atmosphere there. Because of the location, parking and the relaxed atmosphere, I've been to YG several times and have bought a bunch of stuff from them.
I've also been to OGC once, probably lucky to land a front door parking spot when I went. The atmosphere was pretty professional when I went there to look around. I will probably go there again when I have a chance... also to check out Mojo Tactical's gears.
I've also been to the Winward Gun Shop at Hawaii Kai once. I enjoyed talking to the owner and picked up some stuff. Would shop there again.
Lastly, I've been to Sports Authority a few times to browse. Not really sure why but the guy at the counter didn't seem to have much to say. Will probably only go back there if there was a good sale.
I've not been to XRing or the Kaneohe GS yet so no idea about them. Will need to check them out soon!