Reliability is sure enough number one for me too, but I took it that FISwampRat is asking about a gun that I am trying to make up my mind whether or not to buy - not one that I've already bought, took home, and found out was an autojammer.
That said, when I see a gun that I like the looks of in a gun store,
and it feels good in my hand,
and I like the trigger and the sights, I try to resist buying it until I've read some reviews in some gun magazines and on the internet. If I see the slightest hint that the gun is not reliable (and gun writers can be very good at hiding unreliability hints) I don't buy the gun.
Anyway, that's the way I try to do it nowadays. I've made mistakes though. I've bought guns that were jamming pieces of junk, and lost money when I sold them. I'm working on it - I try to not be overcome by temptation as easily as I used to be overcome by it.