Rubber legs are rather new to the fly tying world. Many old standard pattern have had the legs added to them. I have tied and fished rubber leg patterns, with ok success. I don't think they really are more effective than say partridge. Other than you can get a thicker profile of legs with the rubber.
I have been using life flex from hareline and I must say I do like the material, but I truly do prefer natural materials.(other than dubbin, I like synthetics and naturals) I have tied the Golden Oldie with rubber legs, instead of partridge. I gave it a fair test of fishing with it for 2 weeks. The results were low. I didn't catch nearly as many trout with the rubber leg version as I do with the standard version.
A downer of the rubber is that Ive yet to find or hear of any that can last very long.
I welcome and ask for your comments on the subject of rubber legs. Have you found them more effective???