I have a fairly significant family history of colon cancer, including a brother and a cousin who were diagnosed in their mid-40's. As a result, I had my first colonoscopy four years ago when I was 36, and my second one yesterday, a couple weeks after my 41st birthday.
I'm not going to go into details, because I'm naturally squeamish and unwilling to publicly discuss bodily functions--it isn't ladylike, gosh darn it, and I was Raised Right. I will say that I'm glad that between my primary care doc's persuasiveness and my utter terror of cancer (when we say "the C-word" around my house, that's the one we're talking about), I was able to overcome said squeamishness and have the test done.
I'm also not going to lie to you and claim it's an easy process. The prep is pretty much 24 hours of misery. The procedure itself, though, is not so bad, because you're sedated, and at least where I had mine done, they're very kind and understanding about the fact you're scared and feeling like every last shred of dignity you have is being taken away.
But it's still worthwhile if you have a family history like mine or you've turned 50 (the age at which screenings are recommended for everyone). Either you find out you don't have any signs of cancer, as has fortunately been the case for me so far, and you get to go home with one less thing to worry about, or the fact you're having it done as a screening rather than waiting to develop symptoms means they'll be able to catch any issues while they're still at the easily treatable and highly survivable stage. And isn't that worth an uncomfortable and undignified day or two out of your life?