Would be interested in how you are finding it Gary ? Have you encountered any issues ?
I am running JB that I got from herehttp://forum.xda-dev...d.php?t=1704185
I did have some problems with this rom when comming from a custom ICS rom (TurkBey) even with a full wipe, but I then tried it comming from stock ICS and things were much better.
Still having a few issues with it myself. S-Note Lite just does not seem to want to run properly and was giving close messages at strange times. Diasabling it seem to sort this out. With ICS using the stock video player I could click on the 'allshare' icon and it would play the video to my samsung network connected telly. With JB it works but only for about 30 seconds and then allshare seems to crash. Before on ICS I was using an app called 'voice speed dialer' which I could access with double press on the home button. With JB I can not get this app to run via the double home button press any more (and thus not through long press on my bluetotth headset). Also Kies says I can not update with this rom as does the phone itself as it says it is 'modified'.
Overall though other than the small issues above it is running fine for me. It does feel a bit 'smooother' over all though it is not a massive difference to my eyes and mind from ICS. Some things like opening my contacts list is faster as far as I can tell and scrolling on long lists feels faster too.
For me the biggest drawback of JB over ICS right now is the lack of working flash, though this is not down to Google or Samsung really. I did try installing flash from titanium backup but it just crashes on JB using the stock browser or Dolphin. On ICS I could and did use bbc iplayer, ITV player 4oD and a site that uses flowplayer. On JB I can not view video on any of these. I know eventually they will all have to move to some alternative to flash but as of right now my choice seems to be JB with no access to bbc,itv or C4 online video content or ICS with it.
Anyway just thought I would post my experinces so far with JB on S3.