TLDR; The companies that make the reels of ABS are a good shout, the actual printer companies are not a safe bet.
Also a lot of the online companies sculpteo etc. are not public yet despite having a lot of available capital.
Most of the consumer level printers are getting funding through stuff like kickstarter and most of them are fizzling out but contributing back to the larger reprap project.
Investors are invited into this sector unless you go higher up the chain (Parts). 3D printing is an example of how investment can go into the hands of the wealthy for quite a prolonged period of time(the innovation period) before it needs to go public.
Know at which point on the chain of supply you want to invest.
The main investment areas are:
1) Software / Web services (SaaS) stuff IE Sculteo, tinkercad, shapeways..
2) Printer manufacturers (see all the projects on kickstarter etc.) — some companies IE BitsFromBytes are toxic and should be avoided.
4) The electric motor, printer nozzle suppliers
IMHO the smart person longs on ABS or the motor suppliers ~(90%), the risk taker shorts on SaaS ~(30%), the dreamer shorts on hardware ~(5%).. % are my estimation on getting a positive return.
Also remember that amateurs who get good at 3D design are going to want to start using professional tools such as autocad/solidworks and that is another market you might want to look at. Personally I think Blender provides the best value and is the best modelling tool but as it’s FOSS you wont be able to invest with an expectation on return.
Someone who knows more about 3D printing than me might argue that ABS is less quality than the reductive powder available but I’d argue that amateurs wont need such high quality and that the ABS presents a better value case for the next ~10 years.