It is always supply and demand...
Supply is way up with Stern, JJP, and MMR all fighting for those people able and willing to spend approx $7000 on a new game. Most people cannot afford this, and the number of able and willing buyers may be nearing saturation. It is hard to grow the hobby by selling a $7000 machine to a first time pinball buyer.
Older, cheap DMD machines are disappearing into basements, or being restored, fixed up, and sold at higher prices. We used to be able to get a cheaper Data East or so for $750-$1000 at auction, now they are hard to find for less than $2000+. It is a much bigger commitment to get a new person into the hobby nowadays. It is harder to get a new person into the industry for $2000 for his first game versus $750, especially in a poor economy. And for the vast majority, even $750 is a ton of money. Growth is likely tied to inexpensive entry level machines that are still fun and have a lot of action.
I don't want to use the word bubble, I am just not sure pinball is a sustainable industry. My entry into pinball, was based on used, 20 year old machines that had declined in value to a ridiculous level. You could never rebuild those machines for the prices I paid. Most of us got in cheap, enjoying the price discount ($5000 new versus $1000 used) that the arcades and operators ate. I do not see pinball doing well on location around me, at $1 per play I don't know that it makes much money with experienced players. And experienced players seem to be the only ones playing.
I have my ideas where the industry is going, and I could be wrong, but I am gonna stay around for the ride regardless. I love pinball, it is a fun thing for me, my family, and friends, and if my machines lose most of their value so be it, I am not viewing pinball as an investment. I think the next couple of years could be very crazy, my suggestions are to have fun and don't buy more than you can afford to lose.