This is the one hike that all the tourists do, and I can't believe that's the case. I guess let's get the positives out of the way: people do it because there is a very nice view at the top. It's also very close to Waikiki, so it's convenient.
Okay, now let's get to the skinny.
Diamondhead is a big hill of dirt.
Yeah, yeah, it's a big sunken-in volcanic crater. But, honestly, it feels like you're hiking in L.A. There's almost nothing green on the hike - you're exposed to the sun almost the whole way. And you have to fight every tourist, their wife, and five kids to get to the top. And, once you're there to see the panoramic views of Waikiki and the southeast of Oahu, you're there alone with your thoughts and 100 other people snapping pictures, fighting for vantage points, and trying to keep their kids from jumping down.
If you give in to your inner tour guide and decide to do this hike, make sure you do it early in the morning when the crowds are thin and the sun is low. If you're the first one there and you race up to the top, then I would imagine the view all to yourself could be a highlight of the trip. But save yourself the disappointment, and take one of the million other gorgeous and less-crowded hikes on Oahu.