There are so many free scenery mods out there, I'm eager to wet my beak. However, I'm not familiar with how FS interacts with the addon scenery in place of the original MS scenery. I've seen messages from some people who have not been able to properly restore the original MS scenery after removing some custom scenery, without doing a reinstall of FS9. Are most freeware scenery mods easily reversible back to the standard scenery? (As in, just unchecking or deleting the unwanted scenery from the FS scenery screen takes care of everything?)

Yes, in most if not all cases you can just uncheck or delete the scenery if you don't like it.

always always always always review the publisher's readme file. in almost all cases the author will outlines what files are being added or write over stock FS9 files. ALWAYS back-up all files that will/may be changed before the install. if things don't 'uninstall' correctly you'll have the original ready (vs. having to dig things out of the FS9 CDs).(having a redundant backup system is also a good idea -- HD's crash; CDs/DVDs don't write correctly (even though at first they seem OK), etc. etc.--

I don't know about 'easily', but all modifications are reversible if you make backups and keep some notes on what you are doing and what has been done. Easiest are those mods that keep all their files in new scenery and texture subdirectories; those addons that modify existing files can be a pain if they are installed via .exe files rather than allowing the user to copy and track changes. Most difficult are those that make modifications to files like terrain.cfg...DJ

What others have said, plus what I almost always do is to do a forum search on the scenery name and/or author and see what others have experienced. Especially for .exe installs. You will almost always get some hits, positive or negative.Take a system restore point before adding the scenery.Do some of the manual installs first. This will give you an idea of what the usual install procedure(s) is. The .exe installs pretty much do the same thing, except that they self install. If you're familiar with the manual procedures, you will then have an idea of what the self installers are probably doing.