First, the covers.
Next, a peek at the text.
My initial impressions are very favorable. The pages are nicely laid out and the print is large and easy to read. It seems to me, from briefly flipping through all three volumes, that most of the variant readings revolve around spelling. HPL preferred British spellings, but his American publishers tended to change them to conform to American conventions.
You should know that this edition is meant primarily for scholars and ardent admirers of HPL's work. Because the hardback edition is limited to 750 sets, it's rather expensive. I can only assume that there will be a cheaper paperback edition at some point.
But since this is not an annotated edition, I imagine that most readers are likely to stick with, say, their copies of S. T. Joshi's Penguin paperbacks, which contain useful introductions and explanatory notes. The variorum edition is just supposed to collect the most accurate versions of HPL's fictional works, and nothing more.
Anyway, it's good to see HPL's work treated with such seriousness.
Hats off to S. T. Joshi and Hippocampus Press!