I've been running Zeitgeist in 5e for around a year now, and the TL;DR: it's quite a bit of work to do all the conversions.
I think you could easily get away with more reskinning and less conversions than I do, but I like to stay authentic, and our game is already sometimes rather meta that I suspect in my group reskinning would turn into a constant background game of "guess which monster the DM reskinned today" to the mix.
I tend to base conversions on the Pathfinder edition, because they tend to reference common rule elements I don't need to convert (e.g. sneak attack) rather than having lots of unique powers I need to deal with. Also, pathfinder levels translate 1-to-1 which makes that easier. Sometimes patherfinder is just too different to make that work, and then I use the 4e edition since it appears more "explicit".
I think one of the most difficult things is judging difficulty. IMNSHO the 5e CR rules are overcomplicated, and they overstate the extra difficulty of monsters appearing in groups. However, a 5e CR is definitely not a pathfinder CR; I'd say that a 5e CR is at least one step lower than a pathfinder CR - that's generally my guideline to start with.
Oh, and the DM monster creation guidelines are to be taken with a grain of salt. By the book, they generate monsters with way too many hit points and too low AC (just compare a default DMG-created monster with pretty much any monster manual critter).
I also make ample use of
http://www.tabletopping.net/dd-5e-pre-made-characters.html for NPCs.
BTW, for the first few levels you can probably get away with running Pathfinder monsters pretty much as-is, and for a while thereafter by simply reducing AC+attack bonuses+DCs.