Almost all online writing workshops/classes/programs websites have scores of testimonials from students. It is very difficult to understand whether these really are students or paid testimonials. So, do online writing classes really help improve writing skills?
I think it depends on what you're looking for. People take writing classes for several reasons: to receive instruction from a teacher; to meet other writers; to have your work critiqued by a group; to be part of a community of like-minded souls.
So let's break down the answers according to those motivations.
Let's say at the outset that you should choose where to take your class based on the qualifications of the institution. It might be affiliated with a "real" as well as a virtual organization, such as New York Writers Workshop or Gotham. Or it might be affiliated with a major magazine, such as Writer's Digest. If you choose to take an online class, make sure it's with a well known, reputable outfit, and with teachers who have credentials in the kind(s) of writing they teach.
Now for the wanting to receive instruction from a teacher motivation. The way online classes often work is that you have a set amount of time to correspond one-on-one with your teacher, the teacher will read and comment on assignments, and be available during the time of the class.
There are limitations to electronic communication (lack of facial expression, tone of voice, etc.) but there are also advantages to it (people sometimes communicate more clearly in writing) so whether a virtual or physical classroom works best is largely an individual matter. If you're looking for critique and feedback about your work, I'd say that the skill of the teacher matters a lot more than whether you meet face to face or not. However, skill is sometimes more quickly assessed in person--several weeks can go by in an online setting before you know if you and the teacher really "jell".
Moving on to meeting other writers...in physical workshops, the interaction often continues after the class has ended, with members of the group meeting up in a coffee shop to keep workshopping and talking. This is harder to accomplish with an online class, in part because knowing whom you click with can be harder to assess. But it has been done with online classes creating Yahoo groups to continue the interaction.
Group critique is often the most valuable part of taking a writing class--and it's always a hard thing to get, at least when you're talking about good group critique. Some people will have edits to offer no matter what, even if a piece is working well--they're editors and that's what they do. Some people will praise everything; others will dislike everything. Finding people who can really tap into your intent as writer and figure out where your piece hits it or goes awry is like finding gold in the California gold rush. You're as likely to find such people in an online class as a physical one--but without the extra chatting and interaction that happens in a "real" class it can take longer to figure out who's who.
The writers community stuff is basically answered by the above. There are online communities that are at least as stable and long-lasting as physical ones. They can offer support, interaction, an exchange of ideas, and inspiration. The chance that any one class will lead to such a community is small--but so is the chance that one physical class will.
In short, if you're someone who is comfortable online, doesn't require the subtleties of physical interaction, and you can find an excellent outfit, online classes can provide instruction, critique, and even some fellow writers to meet.
Short answer: online writing workshops can be helpful, if you're careful to chose a good one, and if your level of writing and professionalism is in the same rough vicinity as the course. Additionally, almost any workshop has the immediate bonuses of A) encouraging you to write regularly, and B) getting some feedback on your work (though it might not necessarily be helpful feedback). Those are nice bonuses, but often easy to find elsewhere.
Let me expand on those.
Obviously you want a good workshop. You're quite right about testimonials being unreliable - they could be outright fakes, or lacking context, or from people whose opinion you wouldn't respect. So how do you find a good one?
(Personally, I've had a very good experience with Critters.)
Let's talk about appropriate level.
Finally, the bonuses - the very framework of a steady stream of feedback is, for many people, an encouraging, helpful one. This means the group doesn't have to teach you how to write (although it might); it may be enough that it encourages you to put more effort and polish into your writing. And writing is, of course, the best way to learn how to write. So this should be a serious consideration. On the other hand, a workshop can require a lot of effort, and possibly wading through a lot of unhelpful commentary and uninteresting manuscripts. Feedback and persistence can be had elsewhere; workshops are very good sources for them, but hardly exclusive ones.
Oh, and one last important note - one significant bonus you can get from workshops is to hone your own critiquing skills. That's a huge benefit, because it lets you see for yourself what works well and doesn't, and gives you lots of experience editing and suggesting revisions - all of which applies wonderfully to your own work forever after.
I hope these are helpful considerations in deciding whether a workshop can be helpful to you, and which one. Best of luck!
You won't like this answer: It depends.
First: I've never taken a online class. But I know a German online teacher, who led his students to bestsellers. I know this, because the German writing magazines reported on his online course. Yes, now you have to trust the magazines, but well ...
Besides that you should inform yourself about the courses from different sources, it also depends on yourself and how the course is organised. During the above mentioned class, you start writing your novel and send the pieces to the teacher, he revises, sends it back, repeat.
There are a lot of authors out there, who would never show their work in progress to someone else. Not their agent, not their editor, not their spouse. So, not only must the teacher be reputable, his kind of teaching must fit with your kind of writing. Good luck searching.
I asked this question myself. But I gave up researching and continued to write. For me it's a much better writing class.
I'm an online tutor. I don't know that online writing classes "do" help improve writing skills, but I know they can. It's a matter of finding a tutor who is not only skilled, but compatible with your personality. Of course you must decide that for yourself.
As long as you have someone who advises you, and get to write a lot, it will help. Further expanding on the question: You can only get so far with only theory or only practice; theory is necessary: it helps you write in certain ways expressly to create effect. Practice is also necessary: it helps you exercise the mindset you need to write. So the best would be to work with a tutor, write whatever assignments you get and then some more, presenting the assignments and the extra work to the tutor. They can then advise you on things you have done that might not be covered by the tutoring program, or that weren't directly treated in the normal course of the tutoring, thus helping you get farther, faster.
I'm a big supporter of any kind of online writing community. Classes give you a chance to meet others, work with experts, and it gives you deadline pressure to write (which a lot of writers benefit from).
I have provided my top five suggested online writing classes in a post on my blog.