They are a pretty good buy. I can't say if you lucked out getting the 6xx instead not having heard them myself, but personally I think I'd prefer the 58x mostly due to the extended bass. Maybe the 58x has a more "consumer friendly" sound than the other models, but that small bass bump ~100hz can be cheaply and easily modded to ones liking without affecting the extension much.
That said, I think for my preferences, I might say the he4xx would be worth spending the extra $20 on instead. This is going off the impressions I've read that the 4xx is the same as the 400i (if I didn't already own the 400i I would've bought the 4xx for sure). I just like the bigger presentation of the 400i more than the 58x (
@peter123 - you have both (assuming the 4xx and 400i are the same, SQ wise). what do you think is the better value?)