Shortly after I bought my first Canon 1D - the Mark2N, I was walking round with the old (short lived) 100-400IS L on it and was very aware of the weight of the gear round my neck. I bumped into another photographer who had the same gear but who was using a completely different strap. So asking about it he mentioned that these straps had the great effect of spreading the load and making the weight feel significantly less.
We swapped cameras for a minute, and he was quite right.
So I promptly went out and bought myself one and have loved it ever since, using them on body after body. So when I took delivery of the recent 7D and 5D, I quickly ordered another couple.
The latest versions have 'Quick loops' which make it easier to put them on. They come in different colours - I have green on all of mine to help with the camo.
So these come with my recommendation to help spread the light on your gear, and making it feel much lighter.
For US and international readers
For UK readers (with or without quick loops)