For those that are rooted and using custom ROMs, it looks like the devs at XDA have worked out a good majority of the latest OS to the point that it is now my daily driver on my Nex7 and S3 (ATT variant).
Photosphere and some minor video codecs are a hang up on some ROMs, so choose wisely. Liquidsmooth ROM is what I am using on the Nex7 and I haven't had a single issue. I do a lot of tethering and use bluetooth for mouse/keyboard/game controllers/etc. I have tried a lot and this one so far is the best if you want 4.2.1
The ability to add homescreen widgets (you just swipe over one widget and the next one comes up, so it utilizes the same space. You can also create your own widgets now and use them in this area. Just download Zooper app.
Lockscreen also allows for more than just swipe the lock to unlock. You can have up to 8 apps you can unlock to, all in a circle around the padlock image.
Gallery is now a copy of the editorial photo suite Snapseed (with just a few minor options missing). No need to download a photo editor anymore, if you are just wanting to do minor editing. Of course, Photoshop Touch is the go to editor for more complex stuff.
Notifications in your status bar now includes the number of notifications you have for at least text, emails and missed calls. Nice little feature so you always know what you have missed without having to close an app and go to the homescreen.
You now have dual paned notification centers, although one is for toggles and the "User info" for whichever person is using the tablet. You swipe down on the right side of the screen forr this. You swipe down the left side of the screen for the normal notification center.
Multiple Users is now active. I have 3 users for my Nexus 7 now. Each person has their own setup and you can not see the other persons info. It is basically like having 3 tablets. (Downside is, if you buy an app to put on your tablet, only you can see or use the app. Each person has to buy the app, if they want it. This leads to the second downside; you all share the same amount of space on the drive. So if one person uses up 20gb of a 32gb Nexus7, that leaves about 87gb to share between the other two.)
The Google Wallet hack still works for N American S3's, not on Sprint. At least I know for sure for ATT and Canadian Rogers users it does.
One minor tweak I discovered in the Google Play store that used to irk the crap out of me, was when you went into your apps and scrolled down to an app to reinstall it or to delete it permanently, you would get sent back to the top of the list. If you have over 300 apps you have downloaded in Google, this was a time consuming process. A stupid one at that. To my surprise, I downloaded this ROM and while all my apps were auto-reloading back on to my tablet, I went through and stopped certain ones that are larger than 1gb, like NFS: Most Wanted . When I went back to reload certains apps and games, I went right back to where I left off in the list. Nice.
I have also confirmed that Plex, Netflix, and Youtube work without issue. (Hulu hs not updated their code yet, or the ROM I have ismissing the DRM codec. So Hulu does not work. You can browse your content, you just can't play anything.) Youtube got a minor tweak. I don't know if this is a 4.2 OS tweak or a dev tweak, so I will not comment. Needless to say, it is an upgrade.
Nova Prime launcher has also seen an update and it works with 4.2. All icons are now sizeable. You can shrink or enlarge to your hearts content.
Photosphere and some minor video codecs are a hang up on some ROMs, so choose wisely. Liquidsmooth ROM is what I am using on the Nex7 and I haven't had a single issue. I do a lot of tethering and use bluetooth for mouse/keyboard/game controllers/etc. I have tried a lot and this one so far is the best if you want 4.2.1
The ability to add homescreen widgets (you just swipe over one widget and the next one comes up, so it utilizes the same space. You can also create your own widgets now and use them in this area. Just download Zooper app.
Lockscreen also allows for more than just swipe the lock to unlock. You can have up to 8 apps you can unlock to, all in a circle around the padlock image.
Gallery is now a copy of the editorial photo suite Snapseed (with just a few minor options missing). No need to download a photo editor anymore, if you are just wanting to do minor editing. Of course, Photoshop Touch is the go to editor for more complex stuff.
Notifications in your status bar now includes the number of notifications you have for at least text, emails and missed calls. Nice little feature so you always know what you have missed without having to close an app and go to the homescreen.
You now have dual paned notification centers, although one is for toggles and the "User info" for whichever person is using the tablet. You swipe down on the right side of the screen forr this. You swipe down the left side of the screen for the normal notification center.
Multiple Users is now active. I have 3 users for my Nexus 7 now. Each person has their own setup and you can not see the other persons info. It is basically like having 3 tablets. (Downside is, if you buy an app to put on your tablet, only you can see or use the app. Each person has to buy the app, if they want it. This leads to the second downside; you all share the same amount of space on the drive. So if one person uses up 20gb of a 32gb Nexus7, that leaves about 87gb to share between the other two.)
The Google Wallet hack still works for N American S3's, not on Sprint. At least I know for sure for ATT and Canadian Rogers users it does.
One minor tweak I discovered in the Google Play store that used to irk the crap out of me, was when you went into your apps and scrolled down to an app to reinstall it or to delete it permanently, you would get sent back to the top of the list. If you have over 300 apps you have downloaded in Google, this was a time consuming process. A stupid one at that. To my surprise, I downloaded this ROM and while all my apps were auto-reloading back on to my tablet, I went through and stopped certain ones that are larger than 1gb, like NFS: Most Wanted . When I went back to reload certains apps and games, I went right back to where I left off in the list. Nice.
I have also confirmed that Plex, Netflix, and Youtube work without issue. (Hulu hs not updated their code yet, or the ROM I have ismissing the DRM codec. So Hulu does not work. You can browse your content, you just can't play anything.) Youtube got a minor tweak. I don't know if this is a 4.2 OS tweak or a dev tweak, so I will not comment. Needless to say, it is an upgrade.
Nova Prime launcher has also seen an update and it works with 4.2. All icons are now sizeable. You can shrink or enlarge to your hearts content.