They are a great way to help people help themselves, which results in happy clients and regular "semi-passive" income.
Why do I say semi-passive? Because the income you generate is far easier than most, you don't have the hassle of inventory or labor intensive services, and you can get paid many times over for the work you do only once.
But they are not a "get rich quick scheme" it takes planning, and diligent effort to get one going.
Is a membership web site a good idea for you? Well, it depends on a few factors.
What is your topic? You need a topic that is constantly evolving so that your information doesn't get stale.
It also helps if it requires work to maintain the results that the client is looking for (motivation for them to stay in the program).
Or if there are naturally graduated steps beyond the initial accomplishment that a majority of the members will be interested in pursuing.
The whole point of membership web sites is that the member gets the benefit of ongoing support in dealing with a problem or striving toward a goal.
If the problem or goal are a quickly fix, then there is no need for the membership website.
Create an information product for that topic or choose a different one to work with.
Who is your market? Is your market highly motivated to solve this problem or reach this goal? Are they spending money in other areas to do so? Is the problem or goal time sensitive so that if they delay dealing with it, it will become more difficult to do later? Finally, are these people comfortable learning online? At this point most people are, but there are still exceptions.
Make sure that your market will be receptive to the idea of getting this information through an online membership website.
Do you have the expertise? Do you have the personal or professional experience in this topic to make you an expert? Have you had success overcoming challenges in this area? Have you been totally immersed in this topic, so that you have enough information for your average client? Are you willing to commit to continuing your education in this particular area so that you retain your competitive edge? If you are an industry expert, have a continually evolving topic that is important to your clients, and your clients are likely to embrace learning online, then a membership website may be the right business continuity program for you.