Rock tumblers make very popular gifts for adults and kids alike, but it's often difficult to decide which one you should buy. First, you should consider three basic properties:
Rock Hardness. The stones you tumble together must be of the same hardness. For example, you would want to tumble quartz with other quartzes, such as jasper or agate. If you tumble softer rocks with harder ones, the softer rocks will end up in little bits and pieces. In general, rocks with a hardness of less than 5 Mohs are more likely to crumble than polish if tumbled.
Barrel Size. The barrel must be at least 2/3 full, and may be no more than 3/4 full. If you think you'll want to tumble fairly large pieces of rock, you may want to consider one of the larger-barreled models. On the other hand, if you feel that you won't often be able to fill a larger barrel, you might want to look into a smaller one.
Number of Barrels. You may wish to consider a multiple-barrel tumbler. Some of the advantages of more than one barrel are 1) not having to wait for the rocks you found last week to finish tumbling (five weeks from now) before you start tumbling the rocks you found this week; 2) being able to tumble rocks of different hardnesses at the same time; and 3) having a barrel for each member of the family!
For more information, see Comparing Rotary and Vibratory Tumblers.