I have intermittently used Openelec on my 'soon to be' PVR (I hope).
Initially I used it from USB stick, but later put it on the HDD.
Putting it on the HDD was very simple ..... copy two files (KERNEL and SYSTEM) from the downloaded image to the partition and create a menu.lst entry pointing to the KERNEL and also specifying another partition for the configuration files which are stored in hidden directory .xmbc.
I have though become somewhat fed up with the limitations of Openelec ..... so I think I might use a more complete Linux running XMBC instead ..... in the future.
I miss being able to do other things, like multicast live TV from the DTT tuner and some other things too.
Openelec is designed for embedded use, and therefore runs, as best I can describe it, 'live with persistence'.
It is too difficult to install other packages I might want, so not for me I think.
Also, with the tvheadend package installed to manage the DTT tuner stuff it must be administered from a separate PC.
So my view is that this is nice, useful, but very limiting.
Great if it suits your requirements out of box, but if you want to do other stuff, it is not suitable IMO.
It would be great for lower spec hardware I guess.