Before I give my opinion let me just kind of put out there that, not so much in this thread, but in most other threads I've read on this idea, it seems the majority of wrestling fans only have a vague idea of what over/underrated means that basically translates to whether or not they liked the guy in question.
That being said, since the few who've posted overrated haven't really had much argument besides "DDP was horrible" and "he was only out to help himself backstage", general consensus so far is towards underrated.
I think the mere fact that no one has pointed out his feud with Raven is kind of indicative of being underrated. It was probably the best feud of the 90's with awesome mic work going both ways and we're hung up on his hokey feud with goldberg (who IS overrated) without even giving it a second thought. Generally speaking though, he was one of the most over wrestlers in
the company for a long time, spent some time as the top face at the same time as NWO was hot, Sting was in his heyday, and Goldberg was two-moving his way through the roster. When you're at the top and most WCW fans like you, you start moving into the realm of "overrated", but I think the fact that he was so exciting to watch, particularly when you knew the cutter could come out of anywhere, and in comparison to the other top talents at the time (Nash, Goldberg, Hogan, a relative who's who of overrated wrestlers who were just as politcal backstage) it seems wrong to lump him in there. I definitely think in that timeframe though, it's hard to call him underrated since well, everybody liked and respected his abilities in the ring and on the mic.
In WWE he was underutilized and considering he was just as talented as he was in WCW, he was seriously underrated, if not by the WWE fans, at least by the WWE brass. He got squashed by the Undertaker, who like others have said, normally has a pretty good history of putting guys over and did a silly bullshit gimmick with Christian and that was pretty much the long and short of his career in WWE. From what I recalled he was still great on the mic and never put on a bad match. Although, it's a little hard to say exactly how far he could've gone considering his age at the time and injuries piling up, but I'm pretty sure just about every expected a little more out of one of WCW's top faces of the 90's than two dead-end gimmicks.
In overall though, I think when you weigh everything, you got to go with underrated at least in the extent that most people seem to already have forgotten his time with WCW and all people hear about him is from Madden's vitriolic "I hate the f*ck out of everybody" tirades posted on here all the time.