Even if you dislike the REX prefixes of AMD64, you've got to think about the 66/F2/F3 prefixes used for older SIMD instructions. They were introduced by Intel and basically contribute to more opcode bits. There's also the 2E/3E prefixes used for static branch prediction hints in some Pentium 4's (and also in new/upcoming Intel CPUs). VEX is from Intel, EVEX is from Intel, and the upcoming REX2 is also from Intel.
REX is horrible because they occupy two full rows of what were otherwise very useful instructions (inc/dec), and there were other gaps and less-useful instructions (segment override prefixes? If segmentation doesn't work in AMD64 mode, what's the point?) in the opcode map they could've used instead.
