Sadly to say, I've decided to put off leveling my Warlock to test Joana's Guide. I've got other projects that are demanding the majority of my time over World of Warcraft, and I don't really need a sixth high level character.
The guide works, there's no doubt about it. Class really does not seem to matter at the early levels, and after reading through the guide, there's nothing most classes can't solo later on, and the experience gain is going to be more than enough after patch 2.3 to allow you to skip a few of these quests if you can't do them.
If you want to go with a strong solo class, my recommendations are as follows to get the best use out of a solo questing guide (whether Joana's on Horde side or Brian Kopp's on Ally side):
1. Hunter. The guides were written for them, and Hunters are still king of the soloers.
2. Warlock. Once you get sufficient talent points, anything a Hunter can solo should be doable just as easily with a lock.
3. Feral Druid. One of the best classes for soloing tougher content, out of sheer brute force and survivability.
4. Frost Mage/Other spec Mage. Frost mages are great at soloing any non-caster mobs, and have the easiest time IMO when caster mobs come into play. Other mage specs kill fast and still have a decent number of options for kiting.
5. Rogue. One of the classic fastest levelers. They lack the survivability of the above classes in soloing PVE stuff, but really not by much.
6. Everyone else. Every class can solo fairly well, and although I believe the top five classes are going to have the easiest time completing the questing schedule in a leveling guide, with some creativity and knowledge of the class, any class can do well with the guides.
