Mac OS X Lion has been out for a month now in a Developer Preview form. I have been using it since day one. After a month of testing and normal use, I can say with confidence that Lion will blow your mind.
It is fast, smooth and just makes sense. It retains the major features of the Aqua interface but with twists. The iOS influence in Lion is very prevalent but it is not forced down your throat. Gestures used to activate features like LaunchPad and Mission Control just make sense. After a month of using gestures, I am so used to it that I struggle when using my Snow Leopard systems. They're that intuitive.
Another feature that I love in Lion is the Mail application. I used to use Postbox because I love the way it handled Gmail. OS X Lion Mail will crush all third party email applications. It's hard to describe without breaking NDA but I'm sure you've all seen the videos on the web by now. Mail is very much an improved feature of Lion. Applications like calendar and address book also receive new looks but functionality is about the same. A simple application like Mail will sell many Macs by itself.
I was pleasantly surprised by how many applications DO NOT have issues running on Lion. Dropbox was the only application I had to tweak to get it to work on Lion. One line change in a .plist file is all that is required to get Dropbox to work in Lion. Photoshop CS5 Extended and applications of that caliber work fine in Lion. This would indicate to some that Lion is not a major change to the OS other than its looks. I can assure you that is not the case.
Overall, Lion is a major update to OS X. It will be to Snow Leopard what Leopard was to Tiger. I still have not even touched functionalities like AirDrop, at least not with other developers. Lion does have a few bugs at the moment, many of which I have reported. Only one bug I have submitted was ruled a known issue which says to me that they are working hard on fine tuning the OS for the next beta build.
I'm looking forward to seeing what they come up with next.