4. Why should I use it?
Because it's there, and it just works. Maybe you're not a fan of IM or you're wondering whether you really need another IM service. That's ok, this isn't another IM service. It's still Gmail. There's nothing to load up, nothing to download (although the Google Talk client does let you make free calls through your computer). Just sign in to Gmail as you've always done. Maybe you'll start chatting with friends right away. Maybe not. But when you go to write an email and see that friend online, you might just find it easier to send an IM instead. Or maybe when you're traveling and all you have is an internet connection, you'll want to chat with someone back home. Or maybe someone IMs you an important address and you manage to find it later in your saved chats using search. You just never know when Gmail will come in handy.