iOS 6 comes with amazing 200+ new features Facebook integration,Siri for New iPad/iPad 3,Baidu Search,Guided Access, Maps with turn-by-turn navigation and 3D Flyover,Shared Photo Streams, Mail, Passbook.
If you are concerned regarding the compatibility of your idevices with iOS 6 then Apple has released a complete list of devices which will be compatible with iOS 6.
1. Some features may not be available in all countries or all areas. Flyover and turn-by-turn navigation will be available only on iPhone 4S and iPad 2 or later. Cellular data charges may apply.
2. Siri will be available only on iPhone 4S and iPad (3rd generation) and requires Internet access. Siri may not be available in all languages or all areas, and features may vary by area. Cellular data charges may apply.
3. Shared Photo Streams requires iOS 6 on iPhone 4 or later or iPad 2 or later, or a Mac computer with OS X Mountain Lion. An up-to-date browser is required for accessing shared photo streams on the web.
4. FaceTime over a cellular network requires iPhone 4S or iPad (3rd generation) with cellular data capability. Carrier data charges may apply. FaceTime is not available in all countries.
5. VIP list and VIP and Flagged smart mailboxes will be available on iPhone 4 or later and iPad 2 or later.
6. Offline Reading List will be available on iPhone 4 or later and iPad 2 or later.
7. Made for iPhone hearing aids require iPhone 4S.
8. Find My Friends and Find My iPhone enable you to locate iOS devices only when they are on and connected to a registered Wi-Fi network or have an active data plan. Not available in all areas.
iPhone 3GS is also supported in iOS 6 but it will miss out some important features of iOS 6 for sure.
Final iOS 6 will be released to public this fall so it is likely there will be some changes regarding the compatibility of your various devices.