I am enrolled in two courses at the moment, though I already have a BS and am working pretty hard on my PhD, so I am nowhere near caught up. I am treating it less like a class and more like a personal enrichment kind of thing. In other words, I probably won't actually finish the class and won't get the certificate or whatever, but that doesn't bother me. I am saving all the notes and lectures so I can finish them at my leisure when I actually have time to do it. My experience has been very positive so far.
It will, of course, be weak in true engineering classes since engineering really requires a lab component to be truly complete. It's the same reason you don't really see many online bachelor's degrees in engineering, and when you do, they usually aren't from very reputable schools. Still, there are some fantastic computer science and math courses on there right now.