Biwiring: can you really hear the difference?

Very system dependent. I've tried bi-wiring several times in the past with lesser system but did not notice any improvement.

Recently I converted my bi-wired Harmonic Tech Pro 9's with mono terminations to bi-wired terminations and noticed a decent improvement.

I noticed better imaging, 3-D soundstaging, and more air surrounding the highs.

I agree with Stehno, biwiring is system dependant. I originally tried biwiring about 10 years ago, I liked it and stayed with it for about 9 years. Now I'm back to single wiring, why (?), basically because IMHO it is better to buy a better quality cable than split your cable budget on 2 cables. My speaker cable cost about $800 for a 8 ft pair, there is no way I can afford to double that, and it sounds better than the biwire setup I had before ((2) 8 ft cables that cost about $700). If you need them, there are companies out there that make high quality jumpers.
Your experience may vary, depending on many things, such as room conditions, componants, crossover design and the cables themselves. It's definitely best to try before you buy. Take your cable budget and try one expensive pair vs. 2 runs at half that price and see how it sounds in your system.

I've always thought that I'd get better results buying a pair of cables that cost twice as much, rather than two pair. I guess this may only be true over a certain price range. Any thoughts on this?