Caveat:
Have walked it, not played it, have seen it played numerous times in person and (mostly) on TV.
I think by architectural merit it's a good-to-very-good hole. By architectural merit and setting, it strikes me as a great hole.
Although Patrick Mucci does make some good arguments re. its setting, I do think you have to take into account the previous 17 holes (esp. the 17th) when evaluating the architectural merits. The 18th (and the 17th, I'd argue) is a very strong half-par hole, and I'm absolutely sure it plays on the minds of golfers in tourneys as they are making their way around TOC. (I also think it's a superior, superior match-play hole, given the range of scores there, the OB possibilities, its station as the last hole, and other considerations.) There are many ways to play the 18th, a measure I think of any good hole -- flexible approaches to the hole and varying ways to play it. Tiger's approach, for one, is to bomb it as far left as possible, and end up nearly pin high, even if he's 50-75 yards away. He takes the Valley of Sin out of play, and knows he has to hit one very good shot -- his approach pitch/putt -- to make his expected score of birdie. I'm guessing most pros walk off 16 and hope -- for the next two holes -- to score 7, are probably OK with 8, know they are losing ground to the field with 9 or more, and will pick up ground on the field w/ 6.
Most mortals also have a range of options -- easy drive/short iron, force the issue by flirting with the OB right (as in nearly all holes at TOC, the easier approach is from the right side, but going to that side off the tee invites more trouble), bomb a drive and deal with the Valley of Sin w/ a putter. Traditional links weather (all too often missing at the past two Opens at TOC) invites even more choices.
Its setting is nonpareil; I can't imagine even the 16th at Cypress being a more inviting and thrilling place in golf than standing on the 18th tee at TOC.