I installed OS X Lion the day it came out and this was my experience.
The install went relatively smooth from App Store buy process and had to do a couple of reboot but it was up and running just like Snow Leopard when done, except things were totally upgraded under the hood and on some obvious places. While upgrading made things work fine, I saw quite some minor glitches here and there and ended up reinstalling from scratch just to feel better. If you want to do that, you need to create an install disc by following its creation step. (To re-download Lion installer, you need to 'Option' click the 'Purchased' icon, so that it does show 'Install' instead of 'Installed' in greyed out form. And you don't need to copy the dmg file but just drag it to Disk Utility sidebar to start burning it.)
First few things noticed were it had Launchpad and Mission Control app icon appearing in the Application list.
As for Launchpad, this was just like the icon display in iOS but this is just plain not working on OS X as, simply putting the Application folder in the dock folder section is enough to get a good view of all the apps installed and jumping onto another screen just to see it never made sense on screen size of a normal computer. So, I'm guessing this either gets updated to be useful at all or just gets removed since it is redundant.
Mission Control, on the other hand is a nice stretch from the previous Expose and Spaces feature and while it may make some people's life easier, in the end I didn't like it and not really using it except for its Expose part of the features. Mission Control allows to have different desktop spaces as in a space with web browsing, a space with email, a space with image editing but not clutter a single desktop space with all apps open but separate them under different desktop spaces and a nice single swipe on the trackpad can make the space switch relatively easily but this was not good for me as, when I'm working, I have about 6 or so apps open at a time and swiping and watching the desktop space swap with animation takes too long compared to the good old command-tab switch when going back and forth. It may have been better if the space swapping animation can be turned off. I use two displays, one from the laptop and an external one attached to my front, so, I have even less reason to use Mission Control.
Also to a similar feature, there is the 'Full Screen' feature which allows apps that support being full screened to get rid of all the toolbars and docks and become the sole window on the screen (and being able to be swapped with trackpad swipe to other full screen windows). While it may make you concentrate well on it, that didn't happen to me. I feel rather nervous that when rest of the things are hidden... Besides, when under full screen mode, it seems to always take my primary display and shows some empty grey background on the secondary display, even the window was on the second display. And on current implementation the performance has a little problem and it feels somewhat sluggish.
Next big change was how scrolling went up side down... And I think it makes sense that it does feel intuitive when reversed but for many people who might use Windows at work at Mac at home or vice versa end up scrolling in different directions when scrolling to the same directions under those 2 OS... And maybe they will just revert the option back to the previous scrolling model. But the scroll bar being invisible until it needs to show itself is a nice change.
Safari finally can reopen tabs that were open from last session without needing extra extensions by turning such option in the 'General' preference in the system preference. Lion also supports opening up the apps that were open before reboot when rebooting but while it sounds nifty, the apps themselves must be supported to do so and basically I don't find it useful because while it can reopen stuff, it might not necessarily mean to restore the states to the exact state, for example, if you're logged into sites with a user account in Safari, rebooting may not get back you logged in, even if the page gets restored and obviously my third party text editor (MacVim) does not want to reopen all of what was open or my database management app (Sequel Pro) will not reconnect back and show the table I was viewing. So, at this point, reboot is still a reboot which will lose most of what was there on the screen.
The FileVault has took some decent step forward by being able to encrypt the entire disk, which is good for corporate use with good performance. I have written a separate post on this topic.
Finder also made some changes and I will say that Finder is the worst piece of software Apple can create... Everyone (my bet) loves Windows Explorer and uses with ease and I agree with that but Finder is just making life so difficult, I cannot understand how to use this in any comfortable way... The view option, windows position and size seems to reset itself upon occasions and while it can finally sort folders first before files by arranging by 'kind', after 5 years of using Finder, I just cannot feel comfortable when it even resists to have 'cut' feature of files or to create a new empty file (very useful when I just want a simple text file created in a folder) to this day. There are a few efforts to make something better like TotalFinder, Path Finder and those remote file apps like Transmit and ForkLift somewhat do a better job as a file manager too, I wish someone would create something like Windows Explorer before I make one myself out of rage. There was an attempt to create such thing in the past, but it seems the app is buried now. (Funny enough it was created by people called Rage Software.)
Other minor changes include that now FaceTime seems to come free with Lion, so Apple made a nice extra cash for making it non free under Snow Leopard. PhotoBooth becomes so slow when under full screen mode, that it was unusable at this point.
The space usage seems quite low for modern OS as it uses less than 10GB for Lion use and it's probably around 15GB with XCode installed on my machine. So, I still got plenty left with 64GB disk even after having couple Windows OS images for virtual use. Using Monolingual will save about a GB or so for removing redundant language files throughout the system.
For older Macbook Air owners, be warned that CoolBook is not properly supported under Lion at this moment and while removing and installing it again suppresses kernel_task from going crazy, the undervolting setting is not honored and machine may get too hot to put itself in sleep after heavy use, since you cannot throttle it down the way you want.
Generally, OS X is a good OS. It looks sexy, acts fast, third party apps also look sexy and have their own qualities by having affordable price tags but it seems I have failed to gain much from the Lion's new features... I can definitely see the Apple developers took great time to tune to the great details of the OS for Lion. Basically all apps I have seem to work ok under Lion too, so nothing too bad.
Don't miss out on this great long review of Lion by ArsTechnica. It covers so much into such detail, it is an exceptional work.