I wrote a post last year about online personal training. (I even used this same image. Efficiency!) In a nutshell, I'm not convinced it can work well without at least some in-person sessions.
However, it's still a big thing on the internet, largely because it can be easy and profitable for trainers to implement. Still, I was a bit disturbed to see this article directed at personal trainers on the topic of online training. Some of it is fine, but this part in particular bugs me: