I have the rf10 hummingbird, it was a present from the wife last birthday. They are very useful indeed, they show depth in feet or metric, you can alter the fish finder to locate small or large fish, it is also a great guide for finding bottom features or contours. Baz is on the money for the range of up to 30 yards, but that only means you are stumped for distance readings. I use it on lakes and rivers all the time to just give assistance, used on an old telescopic rod with a spare reel it's great.