Sometimes comparing musicians is like comparing athletes from different eras.
Hendrix was definitely great, and as others have said, very influential and ahead of his time. I think that's key.
Jimi had his own gig, like Jaco Pastorius or Earl Scruggs or Thelonius Monk. There were people before them that sort-of did what they were doing, but these guys personalized it and gave it a definite direction in the style of music they played because they had vision and had something to say (which is so important in a player).
Three times in my life I've met people who actually played with Hendrix, so I had to pick their brains to find out exactly what it was about him that made him so special. Most of what I got from those other guys was similar to what Clapton said above.
I'm with the guy who said he used too many effects for his taste. Except, for me, it was that he was so sloppy of a player. All that distortion covers a multitude of sins. Listen to his acoustic guitar playing and try not to cringe.
However, guys like Neil Young and Bob Dylan have singing voices that aren't necessarily traditional or even beautiful, but there are millions (myself included) who will fork over their hard-earned cash to hear them sing live. I've wondered if maybe Jimi is in with guys like that in some ways. His playing wasn't perfect, but it had feeling and emotion and at the time it came out it was very fresh. And it pushed the boundaries out a little more.
People have found fault with an awful lot of Miles Davis's trumpet playing (flubbed notes and half valving), but every time something new came down the pike in jazz, there was Miles leading the parade, being first, showing what was possible.
And like Buddy Holly and Michael Hedges, you have to wonder what he would have done next had he lived. What could Jimi have done if he had lived through the entire 1970's? He would have put out several more albums and I'm sure it would have been delicious.
And, as an aside, I thought Mitch Mitchell, Jimi's drummer, was every bit as talented and fresh as Jimi was. I was originally drawn to Jimi's music not by Hendrix himself, but by the guy dancing on those drums behind him. That was the stuff.