Details, details. Whilst most people are speculating about privacy concerns, spam handling capabilities, and the length of the beta phase of gmail, I'm still busy examining the minutia of the service.
I'll wax opinionated about some of the abstract issues in a moment, but enjoy some more screenshots and commentary first:
The first screenshot shows one of the fun mini-features, namely the green box and arrow to the bottom right of the window indicating what mails are hidden below on the screen. The Yahoo! mail advertisement is styled grey as it's considered part of my signature given that it's below the "-- ". If you omit a standard signature, however, Gmail considers the Yahoo! advert to be quoted text and hilariously replaces it with a small javascript link that toggles its visibility. It usually does this with text quoted in the normal ">" manner.
The second screenshot is what you get when you click on "Compose email", and shows how relevant entries from your personal contacts lists are displayed in real time, updated with each character you type. The JavaScript that drives all of these features is heavily obfuscated, presumably to deter automated interaction with the interface.
One of the most common class of questions that I've had is whether the service is suitable for professional use. Commercial business use is forbidden by the Terms of Use, but if you just want to hide the fact that you're using a free service, then the answer is less clear. Gmail doesn't append advertisements to outgoing emails, and seems unlikely to ever do so, and moreover it may not be against the Terms of Use to send email using a local email client, but that's not specifically addressed. There's an option in the "Settings" section that allows you to set the Reply-To heading, but nothing that will change the From heading—understandably given the nefarious purposes for which spoofing is most often used: spam.
I'm often asked how Gmail works with spam, but I'm afraid I've nothing to report on its spam handling capabilities since I'm being careful to ensure that I get as little as possible! I've had one false positive so far, which was a "Delivery Status Notification (Failure)" from an email sent to one of my accounts with an intermittent DNS, and I promptly marked it as not spam, which is very easy in the interface.
When the Spam folder is empty, it contains the comment "Hooray, no spam here!", and indeed each empty folder has its own little witty remark in it. Even labels can be empty in a sense, as you can have labels that have not yet been associated with any conversations.
I'm relying on the search box heavily, and finding it very useful indeed already even with my small quantity of email, but given that each person has their own different email likes, dislikes, conventions, and workflow, labels will probably be very useful for some. They're allowed to contain spaces, and they apply to entire conversations/threads, not just single messages. The latter property of labels is a little disturbing to me, but it makes sense given that Gmail is highly thread oriented, which is something that takes a little getting used to. I have a feeling that one of the biggest feature requests at first will be to have an option to turn threading off, but that over time people will come to wonder how they ever did without it. It allows for some elegant little UI features, such as the fact that when you reply to the most recent message in a thread, there's no subject editing capabilities by default, but instead there's a link to "edit subject".
Another common question that I've had is as to when the service is going to come out of beta testing. I'm not a Google employee, so I have no idea. With other services such as Spymac apparently starting to offer 1GB of space too, it may be prudent for them to think about doing so soon, but Google's greater media prowess might conceivably offset other services' trying to steal the limelight.
As to privacy, this all comes back to each user's personal requirements. For some, webmail is just not going to be an option for any number of reasons—privacy perhaps foremost amongst them. It's sad that almost every single article on Gmail has been far from objective, missing the fact that everybody has to make up their own minds about whether Gmail is right for them or not. Personally, as you'll've guessed from the euphoria interspersed between my comments on the service, I'm very much enamoured with Gmail, and I find it both useful and very interesting to beta test—but I can accept that many aren't going to use it.
I hope that my elucidation of its features will help people make up their minds a little more objectively, but I suggest waiting to try it out first-hand before settling staunchly on an opinion. You'll probably find, like me, that the more you use it, the more you'll learn about your personal email habits and not just about Gmail itself.