Thought I'd jump in here as I can see both sides of this. I can't recall seeing Kane in a GREAT singles match. I've seen several good ones. I liked the Angle match at Wrestlemania, but on the whole, Kane has always struck me as a guy who is capable of having a great match, but never seems to have one, for whatever reason. He's been in tons of Wrestlecrap angles (Katie Vick, etc.) and as far as I can recall, has never had a legit run in the main event scene, despite being in WWE for 15 years.
His association with The Undertaker has both helped and hindered his career. It helped in the sense that it got him involved in a major player's storylines from time to time, but it hurt because he rarely gets his own stuff. The stuff he does get is crap, angles that can only hurt one's career and I have to speculate that there could be something political behind the scenes that kept his career from ever going anywhere to be stuck with such absolute garbage to work with...seriously, did anyone really expect the Katie Vick thing to succeed? He's essentially Undertaker-lite.
However, after 15 years, his character is about as stale as can be. I can see why no one wants to watch him work anymore. I can't recall him ever having a serious injury and it seems like he's always been there, so he's never been away long enough to get a real overhaul. Hell, I stopped watching wrestling after the Benoit tragedy for a good 3-4 years and I was still sick of Kane when I started watching again.
However, I think he's been a solid guy for the company and I also don't recall coming out of many Kane matches thinking it was the worst thing I'd ever seen.