I've got several WD TV HD's for friends. Quite liked it.
- compact and stylish
- support for lots of codecs
- supports hd
- fast operation (flicking through menus, fast forward and rewind)
- nice looking yet simple menu (some of these players have horrible menus, ie icy box)
- hdmi connection
- lack of hard drive. Imho, any hard drive has a limited life span (having a rotating plate inside it). When it dies, you just need to replace hard drive. If a hard drive was hard wired to media player then you would need to get the whole lot.
- can hook up two external hard drives to it
- supports NTFS for large file sizes.
- it is what it is, a simple media player, so no wireless streaming or any other bells and whistles.
- remote physically quite small (could be a good thing!)
Just make sure to update firmware at some stage to avail of the updates. Easy download from wd site and one click operation.
Haven't had a chance to check out Asus' new media player O!Play, so can' comment on it, but if you do not need networking function, then WD media player is the one to go for. There are actually some nice youtube reviews of it.