You know, I've always been strangely attracted to the P5. I've auditioned them on several occasions and always tell myself that if I can find them for 50% off, then I'd buy them in a heartbeat. They look very classy, are well-built, and are pretty comfortable. They are not the end-all in sound quality, but they have a "safe", versatile sound signature that plays well with most genres and is very forgiving of poor recordings -- which, to me, is more important in an all-around portable headphone than being the last word in resolution.
To the OP, if you've heard them and liked them, then by all means get them and never look back. Are there better choices for sound quality at $300? Definitely yes. But when you play in that price range, you'll likely need amplification and upgraded sources to realize the better sound quality. Rarely are headphones simply plug-and-play.
With that said, the main competitors to the P5 are the headphones featured in armaegis's comparison reviews here: http://www.head-fi.org/t/574617/review-comparison-beyerdynamic-dt1350-t50p-sennheiser-hd25-1-ii-hd25-13-ii-pioneer-hdj-2000-v-moda-m80-aiaiai-tma-1