I like 4e Essentials; I really do. I like the classes; I like the updated monsters. Essentials dragons get two turns in the first round (they roll initiative, then add 10 to the roll for their first action, which means they have a good chance of acting first, even if they roll lower initiative than some or all of the PCs). I like the Red Box and the Twisting Halls adventure. I like the DM screen that came with the DM Kit. I like Mordenkainen's Magnificent Emporium for magic items and treasure.
What I dont particularly like,is the Essentials DM Book, even when you pair it with the Rules Compendium. The Core DM Guide is far superior. There is so much info stacked into the DM Guide--more trap, poison, and disease varieties, how to create your own monsters, how to set up random dungeons and random encounters...examples of symbols to use when designing a dungeon and drawing out a map.
Seriously, one of the basic techniques a DM Guide should include, in my opinion, is how to design a dungeon.
Also, the DMG (as opposed to the Essentials DMB) has all these side bars with tips given by experienced DMs. For example, in the map key for the Town of Fallcrest, #1 is theTower of Waiting. The DMB just mentions it's abandoned. Boring! The DMG has a side note that the tower was used to hold prisoners from the nobility and royalty, and that the ghost of a demon-worshipping princess is said to haunt the tower, and that the DM can design a dungeon beneath the tower. Excellent stuff.
So...if one wanted to go with the Essentials line, my recommendation would be to skip the DM kit, get the Rules Compendium (for the updated math for Difficulty Class and such), but get the Core DM Guide for all the rest of the info a DM would use.