There's also more than one way to slice a rally. On an annualized basis, bitcoin's three-year rise has been slower than the gains seen during several of history's biggest manias -- most notably the Mississippi and South Sea bubbles.
Still, skeptics abound. Howard Wang of New York-based Convoy Investments and Jeremy Grantham of GMO have analyzed bitcoin's advance relative to past frenzies and concluded that it's unsustainable. Grantham, who helps oversee about $74 billion as GMO's chief investment strategist, summed up his concerns in a Jan. 3 letter to investors:
"Having no clear fundamental value and largely unregulated markets, coupled with a storyline conducive to delusions of grandeur, makes this more than anything we can find in the history books the very essence of a bubble," he wrote.
The strategist has a mixed record of success with such warnings. While Grantham was correct to call the 1990s surge in tech stocks a bubble, he exited too soon and missed out of some of the market's biggest gains.
Only time will tell whether Grantham and other bears are right, wrong, or just too early when it comes to bitcoin.