Is Bitcoin a "legitimate" form of currency? Hmmm, I guess that depends on what you mean by "legitimate." Here are the definitions provided by one online dictionary.
1. Being in compliance with the law; lawful: a legitimate business.
I have yet to see a convincing argument that Bitcoin is unlawful (at least under U.S. law). So yeah, by that definition, Bitcoin appears to be legitimate.
2. Being in accordance with established or accepted patterns and standards: legitimate advertising practices.
That one's trickier. In certain respects, Bitcoin operates like any other money. But in others, it is radically different and departs from "established or accepted patterns or standards." Of course, it's those departures that make it so revolutionary.
3. Based on logical reasoning; reasonable: a legitimate solution to the problem.
Oh man, that one's easy. By that definition, Bitcoin is clearly the most "legitimate" currency the world has ever seen.
4. Authentic; genuine: a legitimate complaint.
It's pretty close to impossible to create a counterfeit bitcoin. I'm gonna give it this one.
5. Born of legally married parents: legitimate issue.
I don't know Satoshi's marital status, but I think we can safely assume that this is not the definition most people have in mind when discussing Bitcoin's legitimacy.
6. Of, relating to, or ruling by hereditary right: a legitimate monarch.
Nope, Bitcoin is definitely not that. "Rule by hereditary right" sounds like a better description of the legacy monetary system.
7. Of or relating to drama of high professional quality that excludes burlesque, vaudeville, and some forms of musical comedy: the legitimate theater.
Well, Bitcoin (and this forum) have certainly produced their fair share of drama, but I don't think that's what they're talking about.