I just started Silvia Trkman's foundation class, the first online class I have ever taken. Like Sara said, Silvia is really great about watching everyone's videos and answering everyone's questions pretty quickly. The cost for her classes is really reasonable, considering the price of seminars that are just a couple days of explanation/work/help. The Foundation class is 15 weeks of being able to have her watch your videos and offer you help. And if you want to train with her, this is probably the only way you can unless you can go to her place in Solvenia - I think she's done traveling far and wide for seminars.
How it works with the foundation class, you get a new lesson every 2-3 weeks, so it gives you a good bit of time to work on each lesson. In between the lessons, you post videos of your progress and Silvia responds. Other classmates sometimes chime in too, offering what worked for them. There is a lot of ongoing discussion, way more than what you see in most classes or seminars. The lessons are really pretty intensive but are adaptable to all skill levels and she helps people equally regardless of the level they are at with training. Like some people in the class have absolutely no agility experience starting off and some have trialled at high levels for many years and have dogs who already have a lot of skills. So the former might just be working on say teaching Cik/Cap in the first week and the later might be working on tightening up their Cik/Cap in sequences. Quite a few people are repeaters, so I suspect it takes a lot of people more than one session to work through everything, especially if they are starting with a puppy who hasn't been started on anything yet. She does give a really good discount to people repeating or working second dog. I don't know how other classes work but when you audit her's, you get the same info, same lessons, you can ask questions and join in discussions but you don't post video. So you could still learn quite a lot.
I have heard mixed things about Susan Garrett's classes. Some people love them and some were disappointed. They sure are pricey! I know she has different price levels and the less you pay, the less info/instruction/access you get so that could have something to do with the widely differing opinions I've heard. Don't know anything about Greg Derrett's online classes or Daisy Peel's.