I'm using two MyBooks and streaming to both an Xbox360 and a PS3.
One has ushare and Twonky on (this is my music and TV rips box) and the other just Twonky.
Both manage full 1080p for me, no stuttering problems at all. I am using a gb lan though although given the MyBook doesn't pump out at much speed I doubt that factor has anything to do with it.
The upside of using the two is they have other uses aside from just a media server.
The bad side is neither one is able to handle a massive amount of file formats.
None can handle mkv's which is a shame as almost anything and everything HD comes in MKV format these days. You do get a lot of WMV HD material built specifically for the 360 which is nice, however if you use the same sources as myself they tend to come with Dutch subs hardcoded.
The xbox handles divX and xvid far better than the ps3. So your older material is covered with ease.
The ps3 handles m2ts files which as is fantastic as its incredibly easy to convert an mkv file to an m2ts file with no loss of quality at all.
The ps3 does not handle DTS or VC-1 codec. Converting DTS to ac3 is relativly simple but I've yet to find out how to convert VC-1 to m2ts.
In terms of media streamers I've found on my particular setup ushare falls over when a folder contains a massive amount of files on the 360. It doesn't happen on the ps3, just the 360.
Twonky menu's by default are ghastly, plus its pay for software as well. Easy to setup though (I've wrote a guide myself on this site for the no brainer which when it comes to linux I am!)
Another thing to take into account is the ps3 will not auto insert chapters into large files and the MyBook cannot handle skipping through a high-def movie. This is where the 360 shines, a quick tap of the bumper buttons will skip you through any video by 10% thus if you turn it off for what ever reason its incredibly easy to find the place you left off.
Hope this helps, I'm happy to babble on further if you wish, getting my media hub working properly has been a proper challange, one I've not yet got to perfection!