I am currently taking Rosemary Gladstar's Science and Art of Herbalism correspondence course and Susun Weed's ABC of Herbalism correspondence course.
I began Susun Weed's course about a year ago. At first I was disappointed that there was very little reading material in the course book, mostly only assignments. It began to make more sense once I recieved the herbals I chose and as I began to see how vital it was to actually work with each of the herbs. It wasn't, however, until I listened through Susun's recordings on The Seven Rivers of Healing on Herb Mentor that the course really began to fall into place for me. I highly recommend listening through that or taking the Seven Rivers course at the Wise Woman University before taking the ABC course. Understanding the Seven Rivers of Healing is essential to understanding Susun's approach. One thing I found difficult about the course is the amount of fresh plant material required to do it properly. I have a decent herb garden and was able to grow or wildcraft most of the plants but there were several I was unable to find or grow and that is what is holding me up from finishing the course. Another thing to keep in mind is that it takes quite some time to receive the course materials. Overall, I'm really glad I took this course. If I had to do it over again though I think I would take the Seven Rivers course and then move straight on to the Plant Ally course or the Spirit and Practice of the Wise Woman Tradition course. The reason I say this is that Rosemary Gladstar's course covers a lot of the basic "medicine making" aspects and materia medica and if you are taking that as well it would be most efficient to go straight to understanding Susun's approach to plants and working with them. I intend to take more courses from Susun in the future.
I am almost half way through Rosemary Gladstar's course. It is very thorough and professionally laid out. It has a great balance between science (chemical constituents, anatomy and physiology, etc.)and what I think of as the magic of herbalism. I think both right-brained and left-brained people would enjoy and benefit from it. There is also a good balance between doing research on the herbs and doing hands-on projects. Her directions for the projects are very clear and her recipes are wonderful. I've been using some of her recipes for a couple of years and they are simply the best! I enjoy the homework assignment reports and questions and find it helps me review the material better. It also keeps me moving through the course to have to send in homework periodically. (There is no set time. I just try to send back the next lesson as soon as I receive the previous lesson back which is about five weeks time.) She does utilize helpers to correct the course work so anyone taking it should check over it to make sure any corrections are actually correct. In my first lesson I recieved back a correction that puzzled me because my answer was correct according to the course material. I emailed about it and got back a very helpful reply stating the correction was merely the opinion of the person reviewing my work. It wasn't a big deal but it's something to keep an eye out for. This course does require a siginificant amount of dried herbs. I have an extensive herb pantry but I still spend about $50 a lesson for supplies. I recommend ordering from somewhere you can get the herbs in small amounts. I made the mistake of ordering all my supplies from Mountain Rose Herbs at first and now I have a lot of herbs such as lobelia which I don't plan on using after the course. Now I only bulk order the herbs I plan on using more often and use somewhere like Rosemary's Garden for the other herbs so I can just order an ounce at a time. I highly recommend this course whether you are fairly new to herbs or quite experienced. It might be a little overwhelming to someone completely new but I think if you took it more slowly it would be a good place to start. Even with a lot of experience under your belt, you will get lots of inspiration and new ideas.
I'm also a member of Herb Mentor and have found it invaluable for doing herbal research (you get access to a generous variety of herbs on Herbalpedia) and for continuing my education with herbs. There is a great herbal community there as well. It is an amazing value for the money.
A few year ago I purchased the first lesson of Demetria Clark's Heart of Herbs correspondence course. I wasn't especially impressed with it and never purchased more lessons. It may have improved since then but I thought I would mention my experience with it as well.