I've spent a TON of time over the years researching this topic...first because I was going to buy one for myself and then again a couple of years later as I was purchasing one for my parents to put on my property.
Here's my .02...you have the exact same concerns at 10,000 feet when you are selecting a modular home that you do when having one stick built. There are good manufacturers/contractors and bad ones, you can end up with a dandy or a real lemon, and your results will entirely depend on what builder you select and how much time you spend on the details.
There are total crap modular (and mobile) manufacturers, there are middle of the road ones, and there are good ones. In most instances you get what you pay for. I'll second others here...if you're seriously considering buying go for a tour of the factory.
Installation is absolutely key for success. You can take a top of the line best-in-class modular and if your installer is either A) lazy B) incompetent or C) ignorant specific to what needs done during setup then you're going to hate your house.
My mother has been in hers for just shy of 3 years now...and it's held up very well. But my Dad rode roughshod over the installation process, in several instances making them come back 2 and 3 times to redo areas he was unhappy with.
Make sure your contract has some $$$ in holdback/escrow until completion of all tasks - plus 30 days if possible. That way if something settles/comes apart/etc you have a good chance of seeing fast response time.