Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of 3-D printing start-up ExOne (NASDAQ:XONE), which went public just last week, continue their impressive debut performance with a 7% gain today, after rising nearly 11% on the open.

So what: ExOne has had one of the better debuts in recent memory. On its first day, the stock rose nearly 50%. On the second day of trading, ExOne's shares rose another 20%. Interest has not waned in 3-D printing stocks at all, even as ExOne has earned just $15.9 million in revenue and has endured $11.1 million in adjusted losses for the first nine months of 2012. That annualizes out to $20.2 million, which gives ExOne a price-to-sales ratio of 19.4. For comparison, larger competitors 3D Systems (NYSE:DDD) and Stratasys (NASDAQ:SSYS) have P/S ratios of 11.9 and 9.5, respectively. Unlike ExOne, both are profitable.

Now what: It's clear that 3-D printing mania has not yet run its course. As my colleague Steve Heller pointed out last week, ExOne has only six printers in its backlog, worth a total of $7.8 million. ExOne's focus on metal fabrication helps it stand out somewhat from its peers, but selling million-dollar metal-using printers in the single digits each year isn't going to justify the company's current market cap. Is 3-D printing the new dot-com bubble? Probably not, but the exuberance is starting to look a bit irrational.

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.