I'm thinking about this a lot right now, mostly because I'm contemplating a switch. I've mostly been using the same thing since spring of 2011, but lately I've just been stuck on this shooting plateau that I can't seem to bust through. The frustration of banging my head against the same wall over and over again has really been sucking the fun out of shooting.
So yesterday I tried a different gun and experienced almost instant performance gains. I've read enough threads where this has happened to believe that the "honeymoon period" with a new gun is a very real phenomenon, but the big thing that I noticed is that I really wanted to practice more again. I'm seriously considering doing a full on switch, even if it's just for a couple months, in order to see how this all shakes out.
I figure if I get tired of my new fling, my old gun will still be right there waiting for me, and I may get over this hump for good.
FWIW, old guns are Beretta 92's, new fling is a G34 with a set of Dawson sights and KKM barrel that I bought lightly used from a friend.