I have installed Lion on 4 Macs (2007 Macbook, early 2009
iMac, 2010 MacMini, 2011 13" i7 Macbook Pro) and only
had an issue with my Mini which was quicly resolved with an
update. They all work fine and most of the time faster than SL,
but never slower. Also, working with Lion on a 2011 Mini Server
(came with Lion) and have had no issues. I also note that this
was the first time I had taken the upgrade path vs. clean install.
As with any major upgrade of this nature, back up your current
system. Time Machine is okay but doing a clone of the system
in this case is much better. The clone allows you to boot to the
external drive should something go bad with the install. Also,
it allows you a quick path to return to SL if you decide to do so.
Also, make sure you upgrade all apps to the latest version
and clean out any old "baggage" that you no longer use.
More often than not, people with issues with Lion have some
old app or plug in that has long been forgotten that loads itself
and does not play well with Lion.
Another thing, make sure the disk is in good shape that you are
installing to. Run Disk Repair and Repair Permissions (some
will debate the usefulness of the second, but it won't hurt). Also,
if possible, run some software like Disk Warrior or such that can
search for corrupted files. If you really want to be cautious, also
run some sort of software that can do a surface scan of the
hard disk to identify and repair any bad blocks.
The bottom line in all this is make sure you have a machine
that has no issues or problems before installing Lion. If there
are, Lion may or will only make those issues worse.