The addon author themself is the one that is breaching ToU of the API as addons are supposed to be provided free. You will not get in trouble for using it, though there are just as adequate ones that are available at no cost.
Technically the addon is free (you can download and use it without paying) you pay for the guide which is perfectly legit.
I have it and it's quite good, gets me to 85 pretty damn quick with minimal effort, not quite worth the money though. As far as I can tell it's safe and contains no spyware or viruses.
As has been said above though there are several good free guides!
Zygor is very good if you want to level as quickly as possible.
What to expect
The levelling experience goes something like "Go here, get this quest, hand in that quest, set this inn as your home, go here, kill these, find this guy (here's what he looks like), etc etc"... it is very efficient. Some times it feels a little out of order (gettings the quests), but in hindsight you can tell everything they say is in the interest of completing quests in the most efficient manner possible.
It is so efficient that I would hesitate to recommend it to beginners as you will most likely race through zones so fast you will not have the chance to appreciate the lore -but then that is a moot point to others. For me, I've levelled and enjoyed the zones on my main so levelling alts is more of a chore than an experience, and Zygor is a simple way to expediate that process.
The addon is also very good for completionists and seekers of the Loremaster achievement and Title.
It is about $45 for the levelling pack, but the trial (up to level 20) gives you an idea of what you get for your money.
It's also nice that it's fairly self contained, it counts as two addons (in your addon list) and doesn't depend on any other addons. It also comes with it's own updater which also has a small footprint and helps you feel that you've paid for a good service.
Yeah, the website (I thought) looked professional and checkout was secure as any other SSL site so I trusted them. I think the whole experience (and regular updates) has left me so satisfied that I went and purchased the gathering guides -though to be honest, I would admit they are less needed -but it is handy to have the information available in game.
TL:DR -quick and complete levelling, easy install, cpu friendly and doesn't need other addons.
Edited by Yasy on 07/05/12 08:11 (BST)
Just want to throw this in, the above post is very likely one of the attempts at manipulative advertising the providers of this plugin use to bait people. Have a look at their forums and you'll notice a suspiciously large amount of orthographically correct posts that serve as setups for the providers to tell something positive about their product.
Mind you, if you just bought WoW you already spent $65 on the game and $15 on your first month. Would you seriously want to pay $45 for a leveling guide that takes all the fun and exploration out of the game or rather spend the money on three more months you can use to find out stuff yourself?
If time is a concern, be a Paladin or Druid, then spam dungeons as tank or healer and you'll be 85 in no time anyways. If you can't be assed to do that, just fly to the quest hubs, get your quests and do them... WoW implies there's like thousand ways to get around and get stuff done, but in the end it's one linear path to 85 with just minor differences. There isn't much that guide can tell you.
I'd hate to see someone waste their money with fraudulent stuff like this.
TBH i used this guide, i paid for the lvl 80 ones, when cata wasnt out yet, it DOES help, but not worth the money, easier to get free addons which does the same, but i did like this addon, just costs to much in my opinion