I've been using braided UHMWPE (Dyneema/Spectra) fishing line for quite a while, with braided polyester sheath and a drip of rubbery glue at the knots. It works really well and weighs a lot less than the lightest guylines. I've also used Dyneema/Vectran bowstring material (175 lb breaking strength) the same way, also with success. The fishing line, if used for taught-pitched tarps or tents, will gradually creep and the breaking strength will progressively decline (particularly for lighter line, which will always be operating nearer its breaking load). The bowstring material, because it contains about 1/3 Vectran, won't creep.
I've also made mini-whoopie slings with braided UHMWPE fishing line for adjustable guylines, but slippage was a problem (UHMWPE is ridiculously slippery) unless I used an extremely long pass-through, which became difficult to manipulate and negated some of the weight benefit of the system.
I find that tying and untying knots is impractical with this stuff. How have others adjusted their UHMWPE fishing line guylines?