I use almost exclusively straight MM fare.
I only add levels to humanoids and never multi-class any of them.
I do use templated creatures but never multiple templates which I find silly.
I honestly don't think things need piles and piles of mechanical "enhancements" like levels, templates, multiple classes, "bloodlines" and so forth to make an interesting encounter. Often these things don't make a memorable encounter, they don't appeal to any particular legendary creature or set of abilities.
PC1: "yeah you remember that frog-thing we fought in Pete's game in that pit where I got the sunblade?"
PC2: "no, not really. It had wings didn't it?"
PC1: "no I swear it was purple and frog like"
PC2: "whatever, we killed it with acid"
PC1: "uh no... it was immune to acid"
PC2: "no it had *resistance* to acid"
PC1: "didn't it swallow things whole but kept failing to do it?"
PC2: "beats me, I thought it drained blood"
PC1: "no those were the stirges right before"
PC2: "no I'll never forget the stirges.. damn I hate those little flying blood sacs of death. You're getting confused, we had to firewall the cleric to kill the bastards before he died"
PC1: "I know that! I was pretty sure the stupid frog thing didn't drain blood as well."
This is almost word-for-word a conversation I heard between two players. The only reason the players even remembered the "frog thing" was because they had gotten a sunblade out of the deal. They had trouble remembering any of the details of the fight with the multi-templated leveled bloodlined thingamabob.
Meh, to each his own. Situation is more important to me than the nitty gritty mechanics of the monster.