All the sensible cautions and qualifications about personal preference and system fit being duly noted and appreciated, I have found ribbon cables to be a game changer--easily the equivalent of upgrading my speakers, and I already had very decent cables. Soundsmith Fideliums are an amazing value (in high-end terms) and definitely open up entire new vistas of detail and soundstage. @mglik's review is thorough and matched my experience perfectly.
