I have two Black Rapid straps and love them! Originally I bought the RS4 Classic when it first came out. I had to share with my DH. He kept raiding it off of my camera and putting it on his. When the one designed for women came out, I bought one of those. Now we are both happy. They are very, very comfortable and it is so easy to access the camera.
I usually use mine if I'm shooting wide angle. Typically, when I'm out hiking around looking for birds to photograph, I put my telephoto lens on one camera and put that camera on my tripod. Then I put a wide angle lens on another camera and sling it around my neck on the Black Rapid.
Randy uses his when he is shooting football games. He puts his main camera with the 100 - 400 lens on a monopod and he puts the other camera with a 24 - 104 lens on the Black Rapid around his neck. He can pull that second camera up while still holding the monopod and shoot with it when the action gets close.
The only thing I don't like is that the BR strap screws into the camera where the tripod goes. I have a bracket on my camera and use the Really Right Stuff bracket on my tripod, so the Black Rapid strap is in the way. I have to remove it if I'm going to put the camera on the tripod. They might have a work around for that, but I haven't seen it yet.
The owner of my favorite camera shop always says that photographers are in the hobby to cover their addictions for collecting camera bags and camera straps. I am guilty of both ... until I found the Black Rapid strap. The last strap I will ever have to buy ... until they come out with a better version ... can't imagine what that might be. My ONLY issue with it is that the shoulder pad does not stay positioned properly on my shoulder, but even at that, it is still comfortable and secure. I endorse this product without hesitation!
I have 5 different company shoulder straps . You put a grip , 70-200 or 35-350 - an hour later starts getting heavy .
I bought the seat belt cover and have them on 3 cameras .
I also have a harness - not much better .
Right now I have a canon fanny pack that the 28-135 , 70-200 or 35-350 go in and the 17-55 comes off and goes in fanny when I want other lens .