I installed 0.5 Beta 4 today and played with this feature a bit with my Motorola Droid. One thing to note is I don't like to leave alot of ports open on my computer so I was accessing this via SSH Tunneling on a Putty-like Android program called ConnectBot.
Some Observations:
* Refresh button does not always work, not a big deal b/c my browser can refresh
* I had to dis and reconnect my ssh tunnel if I restarted the SABnzbd server on my computer, otherwise it would display a cannot find webpage message
* It seemed to be aware of everything that was going on with the Server, i.e. if I added a NZB from my computer the mobile UI would see it getting downloaded.
One big problem I had was with adding NZB files from the mobile UI, none of the websites I have access to(which includes NZBsrUs, NZBMatrix, Merlin's -- if some wants to shoot me an Newzbin invite I'll gladly test that

--) was really set up to let me copy the URL download link and then paste it into the Mobile UI's add NZB URL box. I did manage to do it for NZBMatrix and NZBsrUs, for NZBMatrix SABnzbd gave me a warning of invalid username and the download failed, for NZBrUs it didn't fail, just stayed in queue indefinitely w/o downloading
ETA: I had better luck downloading the nzb files to my sdcard but there wasn't a way to add nzb via upload in the mobile UI, are there any plans to add this?