We talked about all this in my ELP Discography thread, but I can't help commenting again.
I have mixed feelings about this one. As a single, I suppose it's pretty good, and I can see why people like it.
But this "song" is actually the closing 1/3 of a longer song which is itself only the first part (first "impression") of an even greater work. The full epic was so damned long that it wouldn't fit on one side of the original LP, so the first 2/3 of the First Impression ended up on Side One and the rest went on Side Two. It was a happy side effect of this that "Karn Evil 9 - First Impression, Pt. 2" was about the right length for a single and had all the right ingredients in approximately the right order and proportions.
My introduction to Emerson Lake & Palmer was their live album, on which the First Impression (actually the whole epic) is played straight through, as originally written. I was familiar with it for years before "classic rock" radio stations appeared much later, and when I first heard "Karn Evil 9 - First Impression, Pt. 2" I was amazed and confused. Why would they play "part of a song"? But at the same time, the answer was obvious. I had Brain Salad Surgery and knew how they'd cut up "Karn Evil 9" to make it fit, and I'd heard this "song" many times, after flipping the record over. And yeah, it was about the right length and composition for a single.
But to me, it will always be "part of a song", the closing part. It contains the shorter recapitulation of the guitar theme, the introduction of which most people haven't heard, and the simpler recapitulation of the synth hook (the middle triplet is omitted) which most people haven't heard. And of course, it's not all of the verses. So it just sounds weird to me.
Hey, if people like it, that's cool. If it got some people to pick up Brain Salad Surgery or one of the thousands of "greatest hits" compilations and, from there, get into Emerson Lake & Palmer, that's cool, too. But it always bugs me to hear this cut on the radio. It just seems to emphasize the divide between how music is composed and how it has to be served up in bite-sized pieces on the radio.