Daring Fireball pointed out John Siracusa’s review of Leopard. Like Gruber, I haven’t spent much time with the land slide of reviews. I’d prefer to experience the upgrade, myself, once my 5-pack arrives from Amazon rather than succumb to other people’s preferences and biases.
Siracusa, though, has done a phenomenally in-depth tour of Leopard. I’m a bit agnostic to some of the UI gripes but his reasoning is consistent and perceptive, regardless. The kernel details are engrossing especially since you don’t have to be a kernel hacker to appreciate what he’s written. Siracusa does a wonderful job of providing sufficient detail to appreciate where the lower levels of the OS have evolved or stagnated and why.
It is a lengthy read, I’ve spent several hours and am still not done, but I think well worth it if you want to speak about Leopard in an informed fashion relative to other operating systems like Windows and Linux and in contrast to what has come before with OS X.