I've had a few.
They have a dry sound that works good with all kinds of guitars, from Tele's to Jazz-boxes.
The new ones with the "contour" or sonic circuit can sound a little like other amps because you can dial out more midrange.
They are fairly powerful in a small package.
Some have "distortion" circuits in them. Depending on the era, these can be decent to dreadful, though the clean sound will always be the same.
If you have a nice sounding guitar, a Polytone will let you hear it. If your guitar doesn't sound so hot a Polytone will help you hear that too!
They are the other end of the spectrum from say, a Fender tube amp. Polytones aren't chimey or bouncy sounding, and don't add much harmonic content.
If your guitar has texture a Polytone won't eliminate it, but if it's flat sounding a Polytone won't help it either.
I've seen old ones suddenly stop working in the middle of a set from overheating, but I have never personally had any trouble with any of mine over the years, and have owned all eras.