Okay, I just looked a couple things up.
First, if it is really the 4
.5-5.6, then it's one of the ED lenses - the cheaper G version was 4-5.6.
Second, Bjørn Rorslett tried out that cheaper G version
and said it wasn't too bad, all in all. So that runs counter to what I'd heard (and posted earlier), but frankly Bjørn knows a lot more than I do.
As far as the "just because it isn't a 2.8" question: Many mid-range consumer zooms can still give you excellent results as long as you understand their strengths and weaknesses. Usually this simply means you get best results if you stop them down a bit - shoot at f/8 or so and you're golden. Shoot at f/4.5, though, and your results will probably be sub-par. The pro zooms are generally not going to be dramatically better than the mid-range consumer zooms at these middle apertures (I won't say this is always true; just speaking generally). Where the pro zooms really pay off, though, is giving you the ability to shoot wide open and still get great images. Additionally, having a max aperture of f/2.8 means you have a significantly brighter viewfinder image - that can really help you nail the focus quickly (either by providing faster AF or else letting you manually focus much more quickly).