After having to use my PS3 for a couple months after my HTPC’s mobo died I have to say it sucks as an HTPC replacement if you have a moderately large media collection.
600+ DVDs and over 19 TV series on my server and it’s kind of a pain to browse all that on the very simple interface of the PS3. Clearly it was meant to be used with a small selection of media, like from something like a thumb drive/portable HDD.
If you really need to use it google a program called PS3 Media Server. Very nice app for this; make sure your server system is decent enough since it will have to transcode everything to stream it to the PS3.