I don't remember how long it took to download because I was working at the same time. Also, the Mac App Store put the download in the dock and only showed a little progress bar and no information such as total size, completed download, estimated time.
When the download was done, it took me 1h10 to install the new system (an installation window appeared, saying installation would take about 33 minutes, which took slightly more than 40, and then a new window appeared, similar to the first one, indicating installation would take about 20 minutes, which took 30).
And then, everything looked the same. The obvious difference was that the scroll bar of some windows appears at launch and disappears, the bar revealed only when the window is scrolled. At the top right corner of some windows, there is the new icon for "full screen", in case I want my screen real estate consumed by just this one window. Windows are now resize-able by each side and corner (woohoo!). Back in 2004 when I used a mac for the first time I was looking for that feature.
Mission control is the new exposé and virtual screens. It's nicely done. The layout in exposé view is pretty (that is, every window of every program minified and stacked behind the program's icon) and useful: the icon of the program in the foreground of each stack, and then the window(s) opened belonging to that program are stacked. If I put the mouse pointer on a window and click space, I remain in the exposé view but the window maximises and the effect is similar to quick look. Active corners remain active. I had set them up for a particular Exposé action, and I keep using them as before.
The biggest change is natural scrolling on the touchpad. They've unified scrolling on Apple devices, bringing the iPhone and iPad scrolling to the touchpad. I'm still not used to it! As though my brain is cabled to adapt my scrolling direction based on the device. Anyway. When I want to read down a document, I've got to pull it (scroll up), and when I want to read up a document, I've got to push it (scroll down). In the systems preferences, one can choose old-skool scrolling.
I didn't notice any improvement in system memory, cpu and battery consumption; it seems no better and no worse than OS X 10.6.
I gave Mail.app a try for one full day. I set it up with IMAP with the same config I have on my iPhone. It didn't work for me, I'm too used to Opera mail, which I resumed using the next day.
iCal presented me with one disappointment. I don't mind their aesthetic choice of faux-leather and torn paper line below the leather pad, I really miss the left panel that showed the calendars and allowed me to display as many months I could fit in that space. This was convenient to quickly check, uncheck, select + refresh given calendars, and the small months view was convenient when planning, next to the main window in which I showed the current week. They added a year view which is pretty (small months featuring my calendar colours and the more stuff I have on a given day, the darker the colour), but doesn't make up for the loss of the left panel. The calendars I created or I'm subscribed to appear in some popup window when I click "Calendars" at the top left of the iCal window and stays on while I mouse-over, click a cal, refresh, etc, until I click somewhere else.
I almost forgot to install XCode! But I had to because I you run stuff like CVS or make. This took me ages and I even feared it would never complete. The Mac App Store let me download XCode (it used to be, I think, on one of the installation DVDs), put the dl in progress in the dock, and when it was done, I was shown Launchpad. It looks like my iPad welcome screen, with icons of all my apps. I clicked on install XCode, entered my system password and waited, waited. Waited. Something went wrong, it was stuck, I had to force quit the installation, do it again, and wait, wait. I think it took more than a couple hours (by that time, I was busy doing other stuff, like cooking, entertaining guests, eating, so it may have just taken 2 hours).
Some apps like TextEdit have active window bars; if I click on the document title on that bar I see light grey text, for example " - Edited" and if I mouse over, I see an arrow. Click the arrow to lock the file, duplicate the file, revert to last opened version and browse all versions. It might make some use of a my local CVS moot.
Amaya works fine. Quicksilver too.
That's all folks.