I have the Lowrance X-4. It's a good unit and serves it's purpose well for the money spent on it. With the Fish ID turned on, it makes everything between the bottom and the surface a fish. So that feature is not so good. One thing you have to remember about fish finders is the amount of bottom you are seeing. With a 20 degree cone in 10 ft of water, you are only seeing 3.5 ft of the bottom. In 2 ft of water you are only seeing 8 inches. A fish has to pass directly through the center of the cone to be shown as a full arch.
Good sites to read about fish finders..... they are great tools to have, but you have to understand what they can do..... I'm learning that for me fishing my ponds, some that are very shallow, it's really nothing more that a depth gauge, but I do like that aspect too. Nothing worse than sitting in 2ft of murky, stained water thinking you are in 10 ft of water.
http://bassfishing-forbeginners.blogspot.com/2010/12/fish-finder-part-2.html
http://www.fishfinder-store.com/howfifiwo.html