Opponents should be played as intelligent and wise as their stats and racial description is portrayed, no more, no less.
I don't know about the rest of you, but if I manage to slay an Illithid I want to know it's because I had a mix of good ideas, quick thinking and a wee bit of luck, not that it was played dumber than it is. Also I love TuckersKobolds because they're played exactly as they're supposed to be, a race that sucker-punch you repeatedly.
There are situations where playing creatures smart would be an increase in CR, for example if we have a band of 6 hobgoblins that waited until after the entire party had moved out onto the one-man-file rope bridge before shooting them with arrows and starting to hack away at the bridge. Page 50 in the DMG about difficulties. I actually prefer these kinds of encounters because it makes things more interesting. But if my gaming group defeats said hobgoblins I would award more xp than if they just had beaten 6 hobgoblins.
There's a huge difference between the DM playing the opponents smart and metagaming. That's not an excuse for DM's to metagame.
As for the original question:
I dislike rocket tag but I like hot-grenade. What the hell to I mean by that?
Well, I like combats that are fast and brutal, because that's how combats are. Dangerous and lethal situations are intense and are more credible. Also as a DM it's hard to keep the suspense and action as well as the immersion going after 8+ rounds.
I dislike if it boils down to win initiative or be halfway to death. I do not know if level appropriate encounters kills you dead in 1-2 rounds, I haven't done the math. It's not been the case when I DM so far, at least so far I can remember. So I like the principle of rocket tag but not the speed of it. So slightly slower rocket tag = hot grenade (playing hot potato with a grenade.). Chucking damage and status effects (including BFC, debuff etc) between the contestants until one of them dies.