iOS 6 has been my operating system for the past few weeks on my 3GS. Here is my review of it. Add yours to this thread!
Speed
Just about as fast as iOS 5, aside from the keyboard bug that is sure to be totally fixed by the GM. Definitely faster than any jailbreak I have ever been on (Even with launchdaemons and languages removed).
Bugs
Keyboard sticks sometimes. But, a bug fix that I was looking forward to in Beta 2 was that the "Share Sheet" in Notification Center was fixed. (Didn't have Twitter and Facebook logos on buttons on 3GS.)
Podcasts were not downloading, now they are thanks to the stand alone podcast app (Get it now from the App Store!) No new text on blue ribbon in Beta 2, if you want to get picky. Do Not Disturb Mode did not work in Beta 1. Single App Mode did not work in Beta 1.
Design
Love the new App Store, iTunes Store, and Music redesign! Phone, not so much. Looks tacky. The grey/blue status bar is horrible. Stick with black Apple!
Functionality
Everything works! It is great!
Features
Maps: Even on the 3GS, where users did not get Flyover or Turn-by-Turn (3GS gets a repressed version of it showing a bird's eye view and directions)
I already love it more than Google Maps. I like how it loads faster, I like the street sign interface better, and overall, (as Steve Jobs used to say) it just works.
Facebook Integration: Working as advertised, although I don't have much use for it, I just use the share sheet.
Single App Mode: Now works in Beta 2! Very useful for helping kids learn, or showing a demo of your app off. Only complaint I have is when you try to exit, a message pops up saying "Guided Access is enabled" Triple tap the home button to exit" Maybe Apple will make it so nothing pops up when you try to exit. Also, you can hard reset the phone to get out of Guided Access. I wish they hadn't enabled that because honestly, kids using iPads in class will figure this out, which sort of distroys the purpose of it. But, I understand that you need to be able to hard reset the phone if something goes wrong, I honestly think if you are truly using it for schools, if something goes wrong, you have to put it in DFU mode and restore it.
Passbook: Should be useful once companies start implementing it. Love the idea, especially with the rumors of the next-gen iPhone having NFC!
Do Not Disturb mode: Really useful feature, as I charge my iPhone by my bed. When you call your number, it doesn't ring on the iPhone, but does show that you have missed a call.
That's about it! I think some of the other features could have been added to the 3GS like Mail VIP's and Offline Reading list, but Apple has got to sell products, and the 3GS is just about out the door with the iPhone 5 coming down the pike.
Summary
All in all, a pretty great release for iOS, and I look forward to the GM and consumer release to seal the deal, although this is a pretty stable beta release.
Speed
Just about as fast as iOS 5, aside from the keyboard bug that is sure to be totally fixed by the GM. Definitely faster than any jailbreak I have ever been on (Even with launchdaemons and languages removed).
Bugs
Keyboard sticks sometimes. But, a bug fix that I was looking forward to in Beta 2 was that the "Share Sheet" in Notification Center was fixed. (Didn't have Twitter and Facebook logos on buttons on 3GS.)


Podcasts were not downloading, now they are thanks to the stand alone podcast app (Get it now from the App Store!) No new text on blue ribbon in Beta 2, if you want to get picky. Do Not Disturb Mode did not work in Beta 1. Single App Mode did not work in Beta 1.
Design
Love the new App Store, iTunes Store, and Music redesign! Phone, not so much. Looks tacky. The grey/blue status bar is horrible. Stick with black Apple!
Functionality
Everything works! It is great!
Features
Maps: Even on the 3GS, where users did not get Flyover or Turn-by-Turn (3GS gets a repressed version of it showing a bird's eye view and directions)

I already love it more than Google Maps. I like how it loads faster, I like the street sign interface better, and overall, (as Steve Jobs used to say) it just works.
Facebook Integration: Working as advertised, although I don't have much use for it, I just use the share sheet.
Single App Mode: Now works in Beta 2! Very useful for helping kids learn, or showing a demo of your app off. Only complaint I have is when you try to exit, a message pops up saying "Guided Access is enabled" Triple tap the home button to exit" Maybe Apple will make it so nothing pops up when you try to exit. Also, you can hard reset the phone to get out of Guided Access. I wish they hadn't enabled that because honestly, kids using iPads in class will figure this out, which sort of distroys the purpose of it. But, I understand that you need to be able to hard reset the phone if something goes wrong, I honestly think if you are truly using it for schools, if something goes wrong, you have to put it in DFU mode and restore it.
Passbook: Should be useful once companies start implementing it. Love the idea, especially with the rumors of the next-gen iPhone having NFC!
Do Not Disturb mode: Really useful feature, as I charge my iPhone by my bed. When you call your number, it doesn't ring on the iPhone, but does show that you have missed a call.
That's about it! I think some of the other features could have been added to the 3GS like Mail VIP's and Offline Reading list, but Apple has got to sell products, and the 3GS is just about out the door with the iPhone 5 coming down the pike.
Summary
All in all, a pretty great release for iOS, and I look forward to the GM and consumer release to seal the deal, although this is a pretty stable beta release.