I've been using a Black Rapid RS-5 for a couple of years now.
Weight wise it is great, and in my opinion the best way to carry around a 5D MK II and a 70-200 f/2.8 IS all day.
However, if you are thinking of buying a Black Rapid, make sure you buy the strap that is right for you.
I went for the RS-5 because I thought with the pouches I could carry batteries, memory cards, etc. and be running around without a bag. However if I'm using the strap while carrying my camera backpack the pouches over the shoulder get in the way.
I also find that if I'm using the phone pouch to hold my iPhone and the camera is held up to my due to shoot, then the padded pouch part of the strap can slip forward. I think the solution is to buy a Brad modifier to lock it in place.
At the time I bought my strap I think only the 4 and 5 were available. If I was buying now I think I would go for a 7 and add/remove pouches as required.