I haven't used one, so I can't say anything about the sound quality,
but one hard disk based player nobody has mentioned yet is the Rio
Karma.

This is indeed another serious contender. Unfortunately I haven't seen
any serious review (i.e. non purely technical) of that player yet.

I really wonder what the sound quality is like.

It supports more formats than most any
other player, including my favorite format, FLAC, which is lossless
and therefore could be useful if you are concerned about sound
quality.

That's a great point. Do you know of other suche formats? It is
unfortunate that the iRivier players do not support it.

The only down side I've heard is that build quality isn't as
good as it could/should be, and a number of the units have failed.

And it has a strange look too.
I guess competition will be hotting up in the coming months too and
other companies will be launching new products.

Thanks for your help.