The editor is okay, but you'll spend a lot of time sifting through terrible excuses for adventures looking for the gems.
You have the option to play galactic adventures as regular missions in the space stage of spore, so I tried this first. It was going good while my missions were being populated with maxis levels, but as soon as random missions began being pulled from the internet it took a very disappointing turn. Simply put, playing random levels relies on the ability of most players to make generally good levels (yeah right),
After an hour or two of frustration, I took to the forums and discovered the "Missions that are Good" sporecast, where frustrated players were collecting a list of missions that were actually worth the effort of playing. While my game experience greatly improved when I started playing missions exclusively from the list, I would still only characterize the experience as average. The fact is most people just aren't cut out for making games.
You may still enjoy this game though because, even though statistically speaking you'll produce feces, you'll probably have a lot of fun doing it. I spent about 6 to 8 hours working on my mission "Outpost Bob", and overall I found the editor to be extremely easy to use, enough so for me to forgive its pitfalls for little a while.
There are about 8 different types mission goals including things such as : talk, move to, bring object, collect a bunch of objects, kill, and ally. You can use all of the buildings, creatures, and vehicles you created (or found) in spore, and there are a number of basic and advanced behaviors to do the most obvious types of things. Everything is easy to use and find and there is tutorial in game to walk you through the basics with advanced videos on the website.
However, if you're an engineer(like me) or an artist of any kind you will probably be annoyed by the lack of detailed controls. You probably won't succeed at framing your artistic vision in this game, and once you played with all of the features the editor will probably lose its appeal.