A fish finder is a valuable tool. It helps in so many ways.
1. It shows depth, giving you an idea of bottom contour.
2. It shows fish. All fish finders are subject to false reading and may indicate fish where there are not any and not indicate fish where there are fish. Various things in the water can lead to false readings. Or even a dirty transducer.
3. Cone width is important. A narrow cone tends to be more precise, while a wider cone can be a little more deceiving. Fish that are on the outside edge of the cone and further away tend to show larger than they are.
4. Most fish finders have temperature, valuable because certain fish occupy different water temps and different times of the year.
5. Speedometer. Not all have speedometers. It is valuable for measuring your trolling speed. Helps being very precise.
6. Shallow water use. The work but the situation is at the bottom of your sonar cone you might only be showing a foot or less. The area shown on your screen is dependent on depth.
I use a Piranha Max 20. Bought it a year ago, I use a gps to measure speed. It has temp, depth, various settings for sensitivity, contrast, alarms etc. It is inexpensive and works well for me. However, if you want the best there are others that show everything under the sun. Like Ken's with an integrated gps.
I would not recommend buying the cheapest model available, in this case you get what you pay for.
So in essence. I recommend having a fish finder. Just decide what features you want and need and find the best on in your budget.
Hope this helps.