'm giving it 5 stars because it does what it is supposed to do in its intended application, that is charging "plain old everyday wet cell starter batteries"
I have several of these, have had them, or similar for many years and they will keep your batteries trickle charged if your vehicle is sitting around doing nothing.
Even a 40 year old John Deere tractor will start up first thing if sitting in the barn all winter with one of these on the battery and a magnetic "stick to the oil sump" block heater attached - been there, done that.

However.....

This trickle charger is not meant for SLA, AGM or gel-cell batteries and should not be used with them. Many later model ATVs, motorcycles and other vehicles use SLA/AGM batteries. Proper SLA/AGM chargers use a 3-stage method for charging and maintenance, and the output is electronically monitored and charge parameters adjusted accordingly. An AGM battery was supplied with my lawn tractor (the instructions are quite explicit in that if charged beyond these parameters, the battery will NOT be covered) Printed on the 20AH battery:

*BATTERY MUST BE CHARGED USING 3- CHARGER*
Step 1: Constant current charge ~20% of rated AH of battery (until current is low enough to signal charger to go to the next step...)
Step 2: Constant Voltage 14.7 - 14.9 (again, until the charger senses minimal current draw)
Step 3: Floating voltage 13.7-13.9, converting current 500-600mA

The comments above that are in parentheses are mine to show the action of the electronically controlled charger. This type of charger switches to each of it's modes based on the final result of the previous mode. After it reaches the floating stage, it stays in that condition unless it is unplugged or reset.

The charger for sale above is a simple limited-current charger that probably starts out at 15 or 16 volts, maybe as high as 18, like the one I use on my car.
Smaller sealed, gel-electrolyte batteries are fairly fragile compared to good old fashioned wet led acid batteries and thus require a bit more pampering. I've got SLA's in some communications gear that are nearly full-spec after 15+ years of regular use, but they are charged using AGM/SLA specific electronic chargers.

Also, some brands such as Optima, etc batteries are AGM BUT they are bit more rugged and can stand being charged by a vehicle's charging system. But never ever attempt to use a simple current-limited charger such as this on anything other than a wet-cell battery. Yes, it is current limited, but usually the voltage is fairly high. I've seen some smaller inexpensive chargers put out 18-20 volts with no load. A wet cell battery will take it (most "maintenance free" auto and truck batteries are wet cell) but even an optima can't stand it, nor can most AGM batteries. Try to put over 16 volts on a 12 volt AGM for any length of time, and you'll find yourself in the market for a new battery.

This advice is from "lessons learned" over several decades with several companies that use both wet and gel-cell batteries. There are FAQs on web sites of most battery companies AGM/gel-cell batteries.

This is a good charger, as I said I have one and have had similar. Very very good for keeping your batteries going after sitting around all winter. The $4 sale price is cheap insurance, and I'd highly recommend them since they are *good quality* trickle charger/maintainers, and even at regular price they cost less than the competition.

BUT as I babbled above, always use the right tool for the job.