As others have said, there are free alternatives... which is important to consider if the price of the Zygor Guides are a concern. If speed and convenience are more important, then they're worth it. Zygor will take you through the quests in the most efficient way possible, and their customer support and the speed with which they update the guides when changes are made to the game is impressive. I know I went from 80-85 a heck of a lot faster (in /played time) than my co-worker who doesn't have the guides. And we had a bet going to see who would make it first. For me, it was worth it. Your mileage may vary...
I first purchased Zygor about 6 years ago from this day. Back then there was no monthly membership so each guide had to be purchased life time up front. I was a bit cautious to spend that much money on an in-game add-on when there were similar ones that were available for free. Soon after I realized Zygor was one of the "you get what you pay for" examples where the quality of guide was so far superior to the free alternatives I actually ended up buying even more guides than I originally intended. Over the years Zygor has improved in the quality of there guides, as well as now offers guides far beyond just simple questing. I think in the past 6 years I have only reported 1 or 2 bugs to the forums and each time I got an almost immediate response from the Zygor admin account to help me with the issue. I have been playing WoW on and off since the release of Cataclysm and have leveled numerous characters multiple times and without a doubt do I believe Zygor is the only reason I have leveled 80% of the characters I have. The fact that you can now get every single guide for such a small monthly payment is a no-brainer and was the rite path to go down for Zygor. Hopefully the company continues to grow and offer quality guides as it does now.
I use it for a lot of stuff. Works incredibly well when you're leveling your 30th alt and just watching Netflix - it has an arrow that you just follow that decides what quests to do and in what order, sells stuff to the vendor and repairs as soon as you click on it, automatically accepts quests and chooses quest rewards, can even have it set up to equip gear automatically if it's an upgrade.
Instead of zygor for leveling, you can use a lookup-website like wowhead.com for individual quests, complete with tips from other players in the comments section. You can also look up each zone on wowhead or gamepedia.com and get a list of quests in the zone. Gamepedia's quest lists for each zone will be in order. You can also use any of the questing addons on curse.com.
I dont know why i havent posted this earlier but i have been using Zygor for more then 2 years now and i simply love it! Worth every penny i paid for it. Speeds up leveling process and makes it pain free. And when your max level fun is not over and Zygor will help you every step of the way whether its Reputation grind, daily quests or achievements its brilliant.
Starting off at some mid level? Come into the guide from any level, scroll through, find and click the level you want to start with, and the guide will take it from there. Zygor suggests starting the guide a level or two before your level. This is to allow you to work you way into the local quest chains. Just skip the chains for quests you didn’t pick up along the way, or the quests that you already did.
Disclaimer: Part 1: If you buy the guide through the links on this page I do get a cut. Part 2: With the guide I have leveled one Rogue from 70 to 90, another from 1-90 (yeah, two 90 Rogues,) a DK from 55-90, a mage from 1-90, and various others. Family members have leveled an assortment of other characters, some with little gear, others with full heirlooms and Recruit a Friend (3x XP.) So I confess, I like this guide. We all do.