I didn't think I was going to cause so much controversy when I posted. I think I should attend to one or two things.
First, some have mentioned that every good team prepares for Scizor. This is partly true, few teams straight out lose to CB or SD Scizor. But because of U-turn it's not as simple as saying 'I'm prepared for Scizor, pokemon X on my team beats Scizor.' Scizor U-turns out of your counter or Pursuits what's switching out. If I have a layer of hazards up, my Scizor is going to be 'beating' your team no matter what preparation you have. Unless your team has not a single pokemon that Scizor can take advantage of, Scizor is at the very least going to gain me momentum and maximize the advantage of having an early game entry hazard plan (which all offensive teams ought to).
I could expand upon why I think Scizor is so good right now. I could get into specific intricacies of how Scizor breaks down teams and why it's so affective. But it boils down to two things:
1. The previously stated momentum advantage it provides and it's general utility.
2. The state of the metagame which makes it very, very difficult to include more than one steel type on an offensive team. Scizor has the most utility (versatility and wide ranging function) out of all offensive steel types. I cannot stress this enough: it is extremely difficult to play with 2 steel types on offensive team in this metagame, in my own experience. There is so little room and so much to cover.
An argument could be made that if Steel typing is so important and affective, and offense cannot afford to run enough Steels, users should stop using offense. I would argue then that the more Stall and Balance that is used, the better Scizor is. It gets to take advantage of more slow and powerless pokemon to abuse U-turn on. It's almost as if you cannot prepare for Scizor in a meaningful way.
This concludes my specific investigation into Scizor and why I think it should be even more popular than it is. I want to point out that Scizor is actually a very, very good check for itself just as an aside.
Others have very wisely suggested that my criterion for what it is to be 'underrated' is different than theirs. I agree: when I say something is underrated I mean that it is used less than I think it ought to be used, regardless of how much it actually is used.
This is different than a pokemon that is overlooked, or which has some function that is unrecognized by many players. And I think this is a false pretense, at least partly: Smogon has many methodical users who use search engines and such resources to find pokemon that can do uncommon things. They are very creative and if there is a pokemon that can do something significant, it is very likely (imo) that someone has discussed it on IRC. At that point it gets some testing by someone, and we find out whether it's any good or not. The undiscovered gem may be a myth at this point in BW. Many hours have been devoted to learning the games secrets, and 99.9% of effective move-sets and strategies are known and recorded, and many ineffective ones are well documented as well.
I was talking with someone the other day about an item that I never heard of called Ring Target. It causes the bearer to lose all immunities, so if you Trick it onto a Ghost you could Rapid Spin against it. Trick it on to a Ground type and it could be hit by Electric-typed moves. Such an item can only be used effectively by a very special pokemon, like Starmie perhaps. I think that it is NEVER used. A literally unused strategy at this point. It is very inefficient and circumstantial, but still I think it may be underrated. It may have a role on a team that cannot afford to ever miss a rapid spin, such a team would need an extraordinarily reliable rapid spinner, and Trick-Ring Target Starmie may be such a pokemon. Maybe it is only good on one type of team, but instead of seeing 0% Ring Target Starmie usage, I think perhaps .1-1% of Starmie could be effective users of the strategy. Thus it is currently underrated in my estimations because it is used less than I think it ought to be.