I'm midway between these two camps. On the one hand, I have enjoyed stories purely for their transformation descriptions (hell, I've enjoyed a story or two just because of a couple of really well written sentences). On the other hand, a well written plot certainly adds another layer of substantive enjoyment to a story.
Here are my thoughts on this, and I'm very aware that I don't follow my own rules all of the time. First, plot ought to flow naturally from the transformation, not the other way around. If the story seems like a bunch of plot with a transformation kind of stuck in there, it doesn't work. If the plot seems to introduce, explain, and heighten the anticipation, it works great.
My other rule is that I try to avoid letting the plot get in the way of the transformation. In other words, there should be a point, text wise, where the plot stops and the transformation is allowed to happen in full bloom, uninterrupted.
What are your thoughts on stories in general and my stories in particular? Do you like plot? Do you feel like certain elements of plot are essential for building up to a good story? What would you like me to concentrate on more - plot details or transformation descriptions? In case you are curious, of all the commissions I've done, almost everyone has requested as much detail as possible on the transformations.