Pros: Inexpensive, accurate, it floats, suction cup, no batteries required.
Cons: Can be difficult to read for aging eyes. It's glass, don't drop it.
I wanted an accurate thermometer to keep an eye on my tank temperature. Mostly to adjust and monitor the functioning of my aquarium heater.
The model that I purchased also had a plastic suction cup to secure the thermometer in one place so that it doesn't float around in the tank. I used to have one that was free floating and it was sometime hard to get at, which often necessitated removing the aquarium top.
A nice feature of this unit is the green printed safe zone range on the termperature scale of 68 to 84 degrees F or 18 to 28 degrees C. The actual safe temperature of course depends on the type of fish that you are keeping and their specific requirements.
The mercury level is hard to read at some angles, but turn the thermometer slightly and you can find a view of the band that appears slightly broader. The lines are very fine for some or our aging eyes, so you may require reading glasses to view the temperature on the scale as well.
The floating feature is great because I like to use it to measure the temperature of my exchange water so it is handy to let it float in the bucket for a few minutes to get a read on the water temperature.
All in all, this little unit does the job that it sets out to do and it doesn't cost a small fortune to get it.