Hey Geren,
I will start this off by saying that I don't own any 55+ communities, but I have looked into them and fully intend at some point on investing in them. I will explain my thought process as to why I believe they're a good investment. I am sure you have likely come to the same conclusions, but I am a firm believer in diversification of knowledge, and hope that maybe I can provide some thoughts that you have (or anyone else reading this) maybe overlooked.
For starters, when I search for manufactured homes in any particular area, one commonality I see is that the well-kept, well-maintained ones are usually in 55+ communities. Why this piques my interest is simple. Manufactured homes are by and large cheaper than brick and mortar. They wear and deteriorate quicker. Now, this is not to say that all 55+ communities are manufactured home lots, but if you consider that the manufactured homes in 55+ communities are better maintained, then you can take that logic and apply it to brick and mortar.
While we could elaborate on why they're better maintained, the bare bones of it boils down to the fact that people living in 55+ communities aren't as rough on the properties as are, say, families with children. Yes, there are exceptions, but we're looking at the by-and-large of it.
Another pro to the 55+ communities resides in the payments. A lot of people living in these communities have social security, they have pensions, they have some form of financial security. Granted, sometimes it's not a lot, but oftentimes it's enough for them to cover rent payments. While there are always concerns when it boils down to money, people in these communities aren't in a position where they can get fired, laid off, or any variant thereof.
As to the purchase options, I generally see a couple routes. Route one, the member buys the house (be it brick and mortar or manufactured) and they then pay the monthly association/HOA/lot fees. I know someone who bought a house (they own the house, not the land) in a community, yet they still have to pay X amount each month for association fees. If they want access to the gym and the pool, that's a yearly fee. Or route two, they rent the house and pay monthly fees as well. (I am sure there are other routes, but those seem to be the main ones.)
With these communities, you want to take into consideration what it is that you offer to justify any monthly fees. Are there exercise facilities? Is there a pool? You would also need to figure out liability and insurance. The fees also go toward you keeping the entire community cleaned and landscaped. Unless, of course, the owners tend to their own property.
Compared to having a single family home that is susceptible to all kinds of abuse, that is tenanted by someone working a dead-end 9 to 5 and what have you, 55+ communities offer a sense of security from an investment standpoint. I have been inside quite a few houses in 55+ communities, and a majority of them look move-in ready, even though that particular person has lived there for 10+ years. There are always exceptions, but the law of large numbers suggests that it's a good bet.
I am sure that someone else will be able to come along and give you some more details, but I felt inclined to toss out a couple ideas.