The much awaited Gmail update is finally out (to selected IE 7.0 and Firefox 2 users yet). After a 43 months long beta-testing, Google has launched v2.0 (Ironically enough, v1.0 was never launched!)Gmail 2.0 would allow users to use the Back Button in browser so that users can bookmark the pages at different sections finally, Thanks to new JavaScript code update. Gmail new version also prefetches all the mails that amounts in less loading time of the messages, especially if its a conversation. (Though people are observing a higher loading time for the inbox and folders that have many messages)
Gmail 2.0 also integrates Picasa Web Album in its interface. Oddly enough though, the Photo picker does not open in the same window, rather opens in a new one. Photo picker is expected to use the available in Google Maps and Google Shared Stuff. Have a look at the new application.
The most significant update in UI is the updated Contact manager (A step forward to become Social Gmail !?). The new contact manager has inline search, making and maintaining a group You can also export the contacts in vCard format, export the contacts from a group and print your address book so you can use it offline. If you want to see the new contact manager and you don't have the new version of Gmail, it can be found here, as its a separate application (but opens in the same Gmail window, unlike the Photo picker)
There is also a new Option button in Gmail Chat Interface. It lets user choose Gtalk photo from user's Picasa web account. Also there are different options for selecting the chat-list size. The hoverover menu has also changed for better User experience.In all, Google has made significant changes in its Newer version of Gmail, but its too early to comment on its performance.
PS - I just had a exception with my Gmail 2.0, when the Internet connection was lost, and Gmail was loading someting!