From DIYinfo.org
The orbital sander is one of the most rewarding of power tools, if for no other reason than that it beats the hell out of sanding by hand, and wearing away your fingertips. It will also give your work a professional touch. For rough work, sanding down old paintwork and so on, you might get by with a sanding attachment to your drill chuck, but for a first-class finish, an orbital sander is the tool to use.
There is little that can go wrong with an orbital sander, but be aware that applying too much pressure can cause the machine to heat up and burn out the electrical armature or wear out the brushes at a faster rate than need be.
So apply light pressure, do not force the work. This will lengthen the life of the machine, sandpaper and give a smoother finish, let the machine do the work. If you choose a model without a sawdust bag wear a good quality dust mask when sanding, some saw dust can be toxic and most paints contain lead and other toxins, which can be hazards to your health.
Apart from the one-handed, small surface machine, orbital sanders are made in two different price brackets. One is built for use in a full-time workshop; normally these machines are in the professional range with a price tag well in the upper price brackets. The other is good enough for the handy man doing an occasional job but is not really a good utility machine for repeated use. These will be in the lower price bracket. Remember you only get what you pay for. There are two common sizes that orbital sanders come in, the ½ sheet and the 1/3 sheet. Abrasive paper and cloth is available in strips to suit the model you buy, but a standard sheet will usually cut into three useable pieces.
A good way of choosing and checking the quality of an orbital sander, is to make a pencil dot on the sandpaper then switch on the machine (sandpaper upwards) and look at the dot. When the machine is running, the pencil dot should look like a circle about 2 mm in diameter. If it wobbles too much, or if the circle keeps changing shape, buy another machine.