Ryobi: The cheapest sander on the market, and for what you get, it is overpriced. Its performance is hampered by strong vibration, which makes it difficult to use for delicate jobs. Not very durable, and makes sense only for people who need a sander only a few times a year.
Craftsman: The quality of their power tools has plummeted over the years. Like most tools, they are outsourced to unnamed Chinese factories working off of a design no one should be proud of. It is never a good idea to use plastic instead of metal for moving parts. Craftsman is either unaware of this, or they just don't care. Even when working right, excessive vibration hampers performance.
Ridgid: They are also now made in China, and can only be bought at Home Depot. Notorious for electrical problems in all of their tools. When they work, they work well. Parts are so hard to come by, many repair shops won't work on them.
DeWalt: Now owned by Stanley-Bostich. The performance of their sanders is fine, however if you are a heavy user, repeated changing of the pad will cause the screw holes to deform, and the sander to become a paperweight. I also had trouble with the power switch. The rubber cover is best removed, as any gunk that gets it their will make it difficult to turn off. And, it can turn on long after you have turned it off.
Milwaukee: Another once respected American tool company has changed hands many times over the last 30 years. Now made by the same company that makes Ryobi. It shows.
Makita: The sander I currently use. Easily the best of the bunch. Quiet, with very little vibration and it is light weight. It is a great tool for me, as I use old wood that has been painted a lot. If I want to get to the third layer of paint, without sanding through to the fourth, Makita has the nimbleness to get the job done. My old man used a Makita. I should have followed in his footsteps.