The Cynic says: M.C. Escher is the artist for people who think Ray Bradbury is literature.
Okay, be that way. Admittedly, you don't have to know or care a lot about art to love Escher. The people who do know a lot have ignored him -- no piles of PhD dissertations, no critical exegeses. To them he's a popular graphic artist, sort of like Peter Max or R. Crumb in the '60s, except that Escher keeps on trucking and trucking.
For instance, for the Escher show that opens today, the National Gallery appears to be expecting a crowd slightly larger than we might expect, say, for an exhibition of Josef Albers's "Homage to the Square."