I upgraded Snow Leopard to Lion (OS X 10.7)

......and it went smoothly and there's quite a bit to like.
I upgraded from Snow Leopard 10.6.8 to Lion 10.7 on my MacBook 6.1 (13-inch late 2009) with 2GB of RAM and a 250GB HDD.
These guides describe essentially what I did; the first is quicker to read and this one is more detailed.
I paid €23.99 for the upgrade which, at the current €:US$ exchange works out at about 15% higher than the USA price.
Download of the 3.74GB installer took about 80 minutes at a download rate of almost 900 kB/s. The subsequent installation actually took an hour even though the installer initially said it only needed 33 minutes.
What I didn't like was the extreme sluggishness of Lion after the first reboot including the dreaded beach ball after even the simplest clicks.
This is due to the reindexing of Spotlight as remarked here.
I left my machine running overnight just to get the indexing completed.
Strangely, I was unable to see how far indexing had progressed either from the Spotlight box or by mousing over the Spotlight icon in the top right corner.
So, after 24 hours, I disabled indexing with the command

sudo mdutil -i off /

then rebooted and now the lethargy mysteriously had gone after which I enabled Spotlight with

sudo mdutil -i on /

I'm still exploring Lion but, so far, it has impressed me particularly with Mission Control and the extended touchpad abilities. Now, I always hated the touchpad and insisted on using a mouse.
But now, for the first time, I'm using a touchpad that is so amazingly versatile and capable that I rarely need to reach for the mouse.
I particularly like the ease with which I can swipe to Dashboard which I've set up like a kind of Conky in OS X.
Mission Control is great if you're doing a lot of stuff in more than one Desktop. With one four-fingered swipe you can see where everything is. Impressive.

I also was able to use a different wallpaper on each of my four Desktops. This posts explains how it can be done. However, when I used this procedure, after a reboot all Desktops had adopted the background of the first Desktop.
Easier is to just swipe to each Desktop, individually, and use System Preferences>Desktop&Screensaver to select a wallpaper for each Desktop.
Doing it this way and the separate wallpapers survive a reboot.

Now this is a bit of an annoyance, but easily overcome.
Many reviews recommend creating a bootable USB or DVD of Lion to enable installation of Lion of any other of your Macs. It might also be useful for recovery of your first (or only, if you have just one) Mac.
However, the installer gets wiped immediately after the install, so if you didn't remember to create your bootable device, you're in trouble, particularly as the App Store shows Lion as being Installed with no option of re-downloading.
But, for this, just Alt-click the Purchased tab in the App store and now you should see Install rather than Installed.