You can safely activate extras.
Extras-devel have apps tested only by the developer and few forum dwellers (or nobody at all ); extras-testing gets apps ready for more structured and deep testing, and if they're found good they go to extras. Apps in extras are quite safe (but bugs are part of life and can't ever be excluded)
If you are not handy with command line and repairing your own device or reflashing, stay far from the first two. If you know what you're doing, more or less, you'll get much more fun having the full range of bells and whistles on.