Most people that replied here are developers or hands-on customers. From my experience, a lot of customers are running 5.2 (with vB4 at least, since it is the minimum requirement), and have not seen a reason to upgrade. Don't get me wrong, there are a lot of goodies in 5.3! But, vBulletin itself is not clean or up-to-date enough to make use of 5.3. For plugins, it becomes a "is this worth the trouble..." thing, at least for me.
I personally use 5.3.6.