I'm planning on buying the HD 500 soon and doing a comparison. I'm impressed with what I've read so far. Even though I'll be comparing, I don't think either product is necessarily better or worse - just different. Each one will have pros and cons.

Line 6 stuff has always struck me as strong in some of the metal/Steve Vai stuff and weak in the clean/semi-dirty department. They have some really nice stereo chorus/delays and wahs and other effects I find pretty useless.

Line 6 hasn't done anything new in a long time, so I'm pleased to see they're still trying to raise the bar.