You're hoping that they're not so devoted to what they have that they ignore what's coming, as Digital Equipment Corp. CEO Ken Olsen famously dismissed personal computers. (Olsen later said he was dismissing digital homes.)
That's always the risk. But match the risk to the potential reward. Note that 3D Systems, which has the longest financial track record, has been growing at nearly 50% per year for some time with rising operating margins. Note that there are bigger companies that could give you a big payday buying these guys out.
I think the risks are worth taking. There are technical traders at 4Traders insisting the next move in this sector is up. Fundamentally, I agree.
Just remember that technology is always a roller coaster, and at some point you may want to get off.
At the time of publication, the author was long DDD.