I've known a number of sci-fi geeks over the years, and it is fascinating to see a whole movie about them. I've never been able to develop that sort of all-encompassing passion for anything, be in the New York Mets or The Grateful Dead, but it's fascinating to see what this obsessiveness produces.
I think Trekkies and sci-fi fans in general consist of people who have been pushed so far out of mainstream society - these are almost invariably the people (like me) who were tortured and marginalized growing up - that they have an almost insane attracting to alternate societies. So they go to sci-fi conventions, joining the Society for Creative Anachronisms (you'll also find that there is a major overlap between fans of Lord of the Rings and Star Trek with people who are into S&M or polyamourous relationships. I wish someone would make a documentary about the whole alternate geek lifestyle thing).
Anyway, this movie is very funny and covers a lot of ground in a pleasantly chaotic fashion, jumping from merchandising to fan fiction to people making it through hard times via their obsession with the show. It is very much worth watching.