I recently graduated from Capella University and got my MBA degree online. The time it takes to complete the MBA is pretty flexible, but I went without any breaks and graduated in 19 months. Overall I feel like the coursework and tests are only somewhat challenging, but quite time consuming. I say this because despite the "blackboard" which is the online posting community, you really do feel like you are somewhat on your own while attending.
I don't mean that my teachers weren't helpful, but without forced interaction with other students, "school" is quite different. Also, because I didn't have study groups like I would in traditional schooling, I tended to over study. Lets be honest though, the reason I chose this type of format was because I wanted to do school on my own schedule, finish ASAP and also because it was much less expensive than my alternatives.
There are some shortcomings in the online setting, but overall I feel like it was a predictable and polished experience. Also, if I was going to get another degree, I would most likely do it in the same fashion.