I'm not a fan. I feel it takes away an important aspect of MMORPGs: The panic (and by proxy, excitement) of aggroing a mob you didn't notice, or didn't know was hostile. Sure, most people will say that it's an annoyance, but I think it's a fundamental part of the experience. It teaches you to not be an mindless drone, and to keep an eye out while traveling.

Now I can just go "Oh, hey, giant red icon. Let's just work our way around that." Where's the fun in that?