I prefer Lowrance, or Eagle (made by lowrance) as a less costly option. I have an eagle fishmark 480 that does an excellent job, and I recently installed a lowrance x-107c that is great, too, with the color screen. It all depends on what you want out of it. If you just want to know depth, most anything will do, but if you are serious about locating fish or structure, then you get what you pay for. The eagle does an super job in grayscale for less than $200. Remember that higher resolution (pixels) is better, as is screen size. I wouldn't personally recommend less than 5" screen size or less than 320 pixel resolution.
Now, if you can spare the cash, I would definitely recommend getting a combined gps/sonar unit. Gps is a great tool, especially for larger lakes, or if you travel around fishing.
Go to
www.lowrance.com and they have great tutorials that may help you decide, even if you go with some other brand. Most of the manufacturers have lots of good info on their sites.
There is a section of this forum with reviews of depth finders, too. You can read what others think of their units.
Take your time to learn about them, make an informed decision to buy what suits
you, and then take the time to read the manual and learn to use it properly. You'll never regret it.