No. Never. Buying leveling guides is a waste of money. Any type of information covered in Zygor is more or less found all over the internet. Leveling is so fast and streamlined now there shouldn't be a reason to use a guide. Go to the zone recommended for your level and do about 20 quests and you're already on to the next zone, outlands, northrend, and cataclysm content is so fast now you can do all 5 or 10 levels of the expansion in one or two zones.
I used Zygor for legion and BFA, used AA on an alt for BFA. Personally i like Zygors format more. I like being able to target things like achievements. I used Zygor to get my Mindworm and my Lucid Nightmare as well as finish the Rep grinds for the allied races. I have zero complaints. My experience was great. I also used Zygor for some professions, recipe acquisition, a couple mounts, all of the Suramar quest line. Thank God for that. And for the last zone of Draenor on my Demon Hunter.. i forget the name.. Whatever the big daily hub thing was called.
We push multiple alpha versions to Curse on an average day, these versions may contain important new features or bug fixes for the bleeding edge raid content... These versions are tagged as alpha, so they will not be downloaded by default. Set your Curse client to update to alpha if you want these latest versions (but they may contain bugs, downloads tagged as "Beta" or "Release" are more stable)
So far it seems like the guides direct you to the easiest/best way to get stuff done. The waypoints they use are better than some other places because it tells you how to get to a certain place rather than saying "Go to X place" it says "Go out of the building and down the path" then once you do those things it says "Go kill X mob Y mob and Z mob until you have A items". It has given some pointers too for an area it was like "If you kill a lava thing it gives you a buff so you can walk on the lava" so it tells you to kill some lava things (JC quest in WoD)
I only subscribed about 2 days ago after coming back to WoW after 6 years. The game has changed so much since I left right around the Cata release. I decided to start fresh on a new server. I still had all my old links and logins to places like Curse, WowInterface, etc. Loaded up all the addons I had favorited years ago (of the maybe 1/3rd that were still maintained). Got to level 20 in a few hours of playing. Had to look a bunch of stuff up.