I don't have any strong view about Essentials as a whole. The Rules Compendium is good, and its DC-by-level chart is crucial, but it's account of skill challenges is underdone (DMG 2 is the pinnacle here).
The Monster Vault is good, although not a replacement for the other monster books.
The Heroes of the F* * books are so-so. I can't remember if any of the powers introduced in those books saw use in our game. Some of the feats did. The martial classes didn't. The general view is that the fighters, at least, are underpowered (especially above Heroic tier). And I think moving from a symmetridcal to assymetrical resource suite is an invitation to needless and pointless intraparty imbalance. When part of the logic of the game is to generate a certain experience around choice of resource expenditure, it seems to contradict that logic to build some classes that don't exemplify that.
You could introudce a class in 5e that doesn't use hit points as its damage mechanism, but some other device (eg M&M-style damage saves), and is nevertheless notionally balanced across the 6-to-8 encounter day. It wouldn't surprise me if some 5e players found that pointless or silly because contrary to a basic precept of the game. The Essentials martials classes look, to me at least, a bit like that.
Of course the later books like Heroes of the Feywild, Heroes of the Elemental Chaos etc are fine - in our game we have a paladin who is a Marshall of Letherna (Heroes of Shadow) and has the Bloodsworn theme (Into the Unknown), a sorcerer who is an Emergent Primordial (Heroes of the Elemental Chaos), and at one point the wizard PC had the cool spell Charm of the Dark Dream (Heroes of the Feywild).
