The Sioux palm drill posted above is top of the line, perfect for aircraft sheet metal work, is small, powerful..a real quality tool. It is my favorite out of every drill motor I have used. The rated speed is ideal for aircraft alum. work,...but for the occasional user you might not like the cost- it all depends on your need. I always buy the best tool, and you only buy it once and take proper care of it and do not abuse it. It will last almost forever.
Note: I only use this for aircraft use. Drilling other things (like steel) I have a host of other drills to use. Drilling aluminum you need 3000 RPM or better, so the 3600 speed of the Sioux is ideal. Steel and other metals require a slow speed, and spinning a drill bit at 3600 rpm in steel will only burn up the bit. Also, the palm sized drills typically have smaller chucks, but for aircraft work 90% of your drill sizes are #40, #30, #21, #10.
Another lower cost drill I have found is this one:
http://www.northerntool.com/shop/too...4122_200384122
This one will perform just as well as the more expensive Sioux, and you can find it as low as $100 or so on-line.
Beware of used drill motors on eBay- most are well worn, abused, may have "wobbly" chucks, etc..