I have a pair of Mirage M5si's. They're bipolar 6-1/2" 2-ways, with matching pairs firing forward and backward. These speakers respond to every improvement in amplification and cabling you can throw at them.
My first wife could IMMEDIATELY tell if they were bi-wired or not. Even if they were biwired poorly, she noticed it within a coupld or seconds. When I didn't have the money for matched biwiring, I combined a set of solid core TARA Labs with some Straightwire 8-conductor ribbon cable. One night after everyone had gone to bed, I fashioned some better-than-factory jumpers from some Vampire Wire. I figured the matching cable should be better than such disparate runs to the woofers and the tweeters.
The next morning, as soon as my wife turned on the system--*to watch TV*--she immediately said, "Change it back; whatever you did to the system, change it back to what it was before." It wasn't even music; it was a talk show over cable, not a $40 audiophile recording, and yet she heard the difference immediately.
A year later, I upgraded the wiring to two pairs of MIT-750 and got a nice improvement in smoothness, linearity, clarity, etc.
Six years after that, Audio Advisor was blowing out the last of PS Audio's foray into premium cables, and I picked up a $1500 pair of XStream Reference Bi-Wire, the only cable up to that time that had been designed from the ground up specifically for bi-wiring. It uses 6-gauge copper for the woofers and multi-strand silver-plated copper for the high frequency input. These made a dramatic difference and were icing on the cake. Before, the M5si's always sounded like big speakers, but the PS Audio bi-wire rig enabled them to scale up and down better, and particularly improved the transparency and intimacy of the presentation. A friend of mine who is no audio geek, but who heard the system before and after the switch remarked how the new cable made the speakers compete with the immediacy and intimacy of minimonitors, which they couldn't do before.
Of course, YMMV. I suspect that when first order crossovers are used, there's less need for bi-wiring, but with more complex crossovers using combinations of 2nd, 3rd, and/or 4th order crossovers, biwiring asserts an advantage.
Biwiring makes a consistently difference with my Mirages, but my Magnepan 1.7s don't offer bi-wire inputs, and frankly, I'm really happy with the sound as-is.