With iOS 6, FaceTime gets a feature that it arguably should have had from day one: FaceTime now works over a cellular connection, not just WiFi. That's assuming you have the right device, and assuming your carrier allows FaceTime over 3G/LTE without making you jump through hoops first. *cough*AT&T*cough*
Even though the iPhone 4 and iPad 2 WiFi + 3G model both have 3G and FaceTime capabilities, for some reason only the third-generation iPad, iPhone 4S and iPhone 5 have been blessed with FaceTime over 3G capabilities. There's been no word why the iPhone 4 and iPad 2 don't get this feature, and it's doubtful they'll get it later down the line.
If you're unfortunate enough to be an AT&T customer, there's yet another artificial obstacle to your FaceTime over cellular bliss: you have to be on a Mobile Share data plan, which are hit or miss as to whether they actually save consumers money or not.
If you're not with AT&T, and your mobile data provider isn't out to deliberately make it difficult to actually use your phone's features on their network, you'll be away laughing on FaceTime after setting the "Use Mobile Data" toggle in FaceTime's settings.
In my own testing, FaceTime over 3G works pretty much the same as it does over WiFi. FaceTime calls get pushed to your device automatically so long as you have coverage; I forgot I even had this activated until I got a FaceTime call from my dad while I was dropping my bike off at the shop. Audio and video quality are both great over 3G, and video quality in particular seems subjectively better than what I get via Skype over the same connection.
I had decent 3G coverage while testing FaceTime on both my iPhone and iPad, so I never experienced any lag, stuttering, dropped calls or other issues. Keep in mind this is a video call from a cafe in central New Zealand made to my father's house in Tennessee. Living in the future is awesome.
Now the big question I'm sure has been on your mind all this time: how much data does all this use? Surprisingly, not as much as you might think. A 47-minute FaceTime call over 3G on my iPad used only about 65 megabytes. That's still over 10 percent of my monthly data allowance on that device (New Zealand telcos are very stingy with their data). However, I'm not going to be making FaceTime calls of that length very often when I'm away from my home or work WiFi, so the hit to my data isn't concerning. Bottom line: don't expect to spend hours upon hours on FaceTime every month without taking a big hit to your data allowance, but short calls here and there aren't a big deal.
Whether or not you feel comfortable taking a FaceTime call in public is up to you, but at least now in iOS 6 you have the option.
Apple iPad Air 2
Apple iPhone 6s