I looked into getting one of these some time back. The main drawbacks seem to be:
Inside, some of the components seem to be very cheap and light (ie: doors, flooring, cabinets, etc.)
#2: they really don't appreciate like a house would. The only thing that will appreciate while you have them is the land that you'll need to buy with it. And typically, the land appreciation isn't nearly as high as with a regular site-built house simply because of the location.
So, while the initial cost can seem quite attractive, just be careful that its meeting your goal of where you want to be in the long run, because it can actually end up costing you a lot in "missed opportunity" of housing appreciation. Case in point, my friend bought a townhouse 2 years ago for $240K and was able to sell it recently for $375k. If he would have decided to buy a manufactured house, he would have missed out on over a hundred thousand dollars.
However, if someone wants to spend considerably less than $240K, oftentimes a manufactured home is the only route to provide them with a NEW, clean, no problem house that is all their own. Sure beats rentin'!