Although it is usually easy to spot a "bald" tire (i.e. one with virtually no tread), you cannot "eyeball" tire tread depth any more than you can "eyeball" the air pressure in your tires. Instead you need the right tool. Although this is one of the more expensive tire tread depth gages on the market, the quality of construction and ease of use justify the price, and definitely merit the five star ratings this product has received. Here is why I like it:1. This tool has a nice heft, is made out of nicely finished metal rather than cheap plastic, and comes with a hard shell foam padded case, which can easily fit into a glove compartment (the dimensions of the closed case in inches are 6 3/8 by 2 7/8 by 1 1/4). The quality construction is evident. This is a very well made tool, and looks better in person than in the picture.2. This is simple to use. Just loosen the lock screw, push the slide so the narrow point on the left is flush with the metal face, turn the unit on, press the zero button (or alternatively just adjust the slide until it reads zero on the digital display), choose whether you want mm, inches in decimals (tenths of an inch) or inches in fractions (64ths of an inch), put the metal face flush on the tire, push the slide from the "ruler" side until the narrow point reaches the bottom of the tread, and see the digital readout. The whole process takes just seconds.I like that you can simultaneously see the reading mechanically on the "ruler" part of the slide, which is marked in mm and 32nds of an inch. The new tires that I buy for my cars typically start at 10/32 of an inch (though this can vary depending on the kind of tire, so you need to check what the tread starts out as on whatever tires you are using). Therefore a reading of 5/32 for me means 50% of my tread is gone, at which point I usually replace the tire.3. Opening the battery compartment and installing the battery are simple. This product comes with the necessary battery (and in fact mine also came with a spare).4. The digital display is easy to read, and the digital reading remains on the screen until you either turn the unit off or "zero it out" to take another reading. Although I wear glasses, I can easily see the display even without them on. The digital display eliminates any guesswork as to the reading, though as noted above, you can also simultaneously look at a mechanical reading just to confirm the digital readout.5. I have my oil changed at a place that indicates the tread depth on each tire, but I bought this unit so that I could check myself between oil changes, and also verify the readings I am given. This tool easily and accurately lets me do that.Bottom line: If you like using quality tools that are easy to use and work effectively, then I recommend this product for checking your tire tread depth. This tool is handy for me to periodically check the wear on my tires, and is also useful for those who like to buy used tires, or are considering buying a used car and want to check the tire tread depth.