I received a tuck of the yellows and blacks in a recent PIF. I've only used one blade of each so far. First impressions are positive.
I would say just pick one, or a sampler pack with as many of the 7:00 series as possible, and see how they work for you. As you can tell from all the comments, what works for one might not work for another.
7 O'Clock greens almost made me quit shaving. Terrible on my skin for some reason. Perfect example of YMMV
Black 7 o'clocks work good for me. Those are made in India. The Russian 7s are not on my list of good blades. In fact no blade made in St. Petersburg with the name Gillette on it is on my good blade list.