Stupid kiddie theme, none of the kiddie themed pins Stern have made in recent times have sold well, a good example is Shrek only sold approx 500 units 
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My understanding is that was actually limited deliberately to clear out 500+ excess blank Family Guy playfields/whitewoods Stern already had manufactured. When FG sales were lower than expected they came up with a "Limited Run" of Shrek to primarily clear inventory. This was just as Stern were heading into their 2 year sales slump.
Regarding Muppets, I do agree it is a strange idea for a theme that is for sure. Just way too niche and quirky, even for a market such as pinball.
I think the Muppets have had a big impact on popular culture and it may not appeal to this forum, but I reckon their general popularity would make this a good Stern pinball seller.
Here's a nice article that sums it up nicely:
http://thatwasjunk.com/2011/12/21/my-favorite-movies-8-the-muppet-moviethe-muppets/As for Family Guy, what I'd give to see a re-run of that machine.....take my money!
Cheers!
Adam.
