"Apple users match up well, because they tend to have creative professions, a similar sense of style, and an appetite for technology."
From a New York Times article about some some new "niche" dating sites, including one solely for owners of Apple computers.
By definition, niche dating sites are not everything-for-everybody, one-stop-shopping kinds of sites. They're for people who want to narrow the universe of potential partners to people who are like themselves in some way they consider important. Thus, if religion is a big part of your life, you can find sites limited to people of your particular faith. If you're a book lover, there are sites where you can search for people with reading tastes that are similar to yours. If you're unlucky enough to have a sexually-transmitted disease, there are dozens of STD sites that allow you to avoid all those worries about what to say and when and how to say it.
There are literally tens of thousands of niche dating sites, some of them so specialized that you wonder how they can attract enough members to stay in business (e.g. stachepassions.com: a site for men with mustaches and the women who are turned on by them). Some niche sites are free, but most of them charge anywhere from $5.00 to $50.00 a month.
I can see the appeal of some of these sites. If you're Mormon, say, or Greek Orthodox, and you're 100% certain you only want to date within your faith, why join a site like match.com where only a tiny percentage of the members meet your requirements? If you don't feel you could date a meat-eater, why not join a vegetarian-only site? If opera is the biggest thing in your life, why waste your time with someone who not only hates it but ridicules it?
All of that makes sense. But it's important to realize that common interests and shared attitudes can only take you so far. For a relationship to succeed, there has to be chemistry between the two people. Having something in common may help break the ice by giving you something to talk about, but at some point a relationship requires more than just a mutual interest in, say, Apple computers or the novels of Jane Austen.
There's also a danger in attributing too much to a particular shared interest. We want to think that if a new person in our life is like us in some way, he or she will be like us in other ways, as well. But oftentimes that's not the case. We might both be dog-lovers, but that doesn't mean we have similar opinions about social or political issues. Or we might have similar opinions about social or political issues, but have incompatible communication styles.
The bottom line is that dating sites---whether niche or mainstream---can only, at most, identify candidates who might be suitable for us. No more, no less. That's why it's crucial that you meet someone you're interested in as soon as possible, and not get over-invested in him or her until then. When you're face-to-face, you can tell more about someone in five minutes than you can in five months' worth of e-mail exchanges. You can see what he actually looks like, how he dresses, how he talks, how (or if) he listens, whether he has any social skills, or, conversely, any annoying habits.
Chemistry may be hard to define, but one thing we know about it is that it can't be willed into existence. It's either there or it isn't, and all the common interests in the world can't create it or sustain it.