I bought Manual of the Planes and have used a few. A bare few.
Some are interesting to read about but don't look like they'd make for fun gaming. I suppose the issue is just that I have several planes I like and some I don't like.
I like whichever Karasuthra (part of a plane). Astral and Shadow are cool (there needs to be more Astral monsters in 3.x/4.x though), Mechanus is half-cool, the Plane of Time was surprisingly cool. If 4e is too my liking I think I'll write something for Karasuthra and Time.
Many of the upper planes are uncool, and so is the Ethereal plane.
Some are too inaccessible, though. Limbo is cool, until the PCs burn up due to random fire-transformation. (Also, drawing a combat map there would be a PitA, even without the lack of gravity.) The Elemental Planes and Positive/Negative Energy Planes are inaccessible; the Negative Energy Plane was used to isolate Athas, in fact. Obviously the Lower Planes are accessible; there are thousands or millions or billions of demons and/or devils and/or yugoloths down there, which means that any adventurers that make the mistake of going down there die... quickly.