Well, I have been trying to decide which I prefer. now that I own both.
I have just realized that it has been about a week and I still can't make up my mind. And this fact alone, shows that perhaps the winner should be the MX500. Costing 4 times less, it is enough to keep up with the 888.
I do like the 888 a lot, especially with classical. Alia Vox recordings for instance, "Harmonie Universelle" are just incredibly detailed. But with some other recordings, I noticed sharper highs, perhaps a bit too bright... with Paquito D'Rivera's "Chucho" on Chesky, that trumpet almost makes me think the E888 exhibit some sibilance.
MX500 do have thicker mids, and while Paquito sounds better, other recordings don't sound as clear.
Bass on the 888 is much better than I expected from the comments here... it is a decent natural bass, not as boomy as with the MX500, but that is perhaps because the 500 seem to seal better. I can't get a perfect seal with the 888, and they lose some bass because of it.
As for comfort, the 888 win BIGTIME. Much more comfortable. Shape is much better, and they weigh less. They fit my ears much more naturally, even with less sealing. The longer wire also helps a long... plus build quality seems better.
Overall, I am not sure I would buy the 888 today. I think I prefer their sound to the MX500's and would classify them as better, but the difference doesn't justify a 4x higher price tag...
For now, I'll keep the 888's in use, inside the MX500's case...
Gotta check those new Aiwas people keep talking about.. and maybe the KSC if I can find them.