This helmet is not overly heavy and has great side vision. My medium weighs 980 grams, almost exactly 3 times my Fox Striker, but much, much safer.
I agree with Nathan below, If in doubt go smaller. I am right at the very top of the medium chart and was a little worried since my head is kind of round, but the medium fits fantastically.
Also, in case you haven't dealt with Backcountry before, these guys are AWESOME! My helmet came in 1 1/2 days to N. CA. Very happy with this webstore!
One reviewer below talked about the carbon layup at the back of the helmet. Yes, it could have better workmanship; you can see sort of a frayed edge seam, but it isn't horrible. My guess is this is one of the reasons it is so cheap now. For the price, I don't see how you could not buy this thing.