I experienced major problems downgrading -- for some reason SL couldn't be reinstalled, and Lion reported that my Time Machine backup file couldn't be verified, so it wanted to create a new backup file from scratch. 
I discovered that if you hold down the Option key while rebooting your computer, a startup option called "Recovery HD" appears. Click on that and you can restore from a Time Machine backup, even if Lion thinks it's a bad file. You don't even need the SL installation disks. (Just make sure to copy beforehand any files you've created or modified since you installed Lion, because you will lose them when you restore to SL.)
Glad to be back with SL. Lion looks promising, but it's still a little too buggy for my taste.
I discovered that if you hold down the Option key while rebooting your computer, a startup option called "Recovery HD" appears. Click on that and you can restore from a Time Machine backup, even if Lion thinks it's a bad file. You don't even need the SL installation disks. (Just make sure to copy beforehand any files you've created or modified since you installed Lion, because you will lose them when you restore to SL.)
Glad to be back with SL. Lion looks promising, but it's still a little too buggy for my taste.