There is no recent update to either the 'User Friendly' or 'Professional' firmware. The ones posted in the downloads section here simply show the date they have been posted online. They are the same builds that shipped with the unit and the ones included on the Installation CD. The problems that exist with the unit (ie Power issues), and others documented at DD-WRT have not been addressed and still EXIST. I haven't looked at the Netgear nor the Linksys unit so I have no opinion on them, but I do own a WZR-HP-AG300H and I would stay clear of it for now. It just doesn't live up to it's claims, and for a 'flagship' product the support is lacking.
Further on that, I'd suggest that you filter the advice you get for ACTUAL OWNERS of this product. Those in here (like some mods) that seem to discount the issues on this device YET DO NOT OWN ONE are suspect IMHO.
I'm currently running the 'User Friendly' FW. It does allow have an implementation of 'Guest'. I have not used it, but I understand that you can create multiple SSID's for each radio (2.4 & 5 Ghz) with it's individual security (WPA, WEP etc.) While setting that up you can 'Separate Feature'
"Separate feature
Connected wireless devices can communicate with WAN side devices if this setting is enabled"
I thought (i could be wrong) that if this is checked then devices connected via wireless can not see other devices either on the same SSID or others nor can they communicate with those devices connected via a LAN Drop.
One item that seems to be missing (and requested within DD-WRT Forums) is an ability to control 'Guest Access' bandwidth. Sharing my connection with my neighbor might be nice, but not once he decides to load up some torrents ;)