Portapro's are amazing in my setup: quarter modded (super simple), with a small E6 amp. With the highs a bit more open as a result of the mod and amp, along with the thump and impact of the bass, I can't imagine anyone not having fun with the portapro's. :)
I can't speak to the quality of the 500's. I just know I was perfectly pleased with the 700's, but thought the hype on head fi was just laughable: exaggerated to a gross degree.
Are the 500's significantly better than the 700's? The 700's are solid, but there's nothing there that should lead to the glowing reviews on head fi and amazon, IMO.