Pros

• to me the felt covering seems a little bit less loose than that of real tennis balls
• they also float pretty good
• signal colored (easier to spot for the dog even not moving; at least that’s my personal experience)
• more expensive than the average real tennis ball, but still at the more affordable end.

Cons

• disappointing bounce (really they hardly bounce, it’s sad!)
• very compressible (risk of choking or swallowing)
• NOT chew proof
• very dangerous small piece (squeaker) included (read more below)! Although some dogs love squeakers, I think the risk outweighs the benefits in this case.

Special note: By far the worst danger these balls pose to my mind is the integrated squeaker!!!

This tiny piece is just slid into a pierced whole in the plastic part of the ball and then covered by some of the felt and some kind of glue.
When Mila compressed the ball after a few minutes of playing and occasional compressing again, the squeaker merely popped out.
Luckily she did not carry this side of the ball in her mouth at that moment. Because I think it could have easily shot right into her windpipe, causing some serious problems.
I found the little thing on the patio floor and examined another one of the balls to see where that came from. I was able to easily remove the squeaker from the other ball just by loosening the felt a little bit and compressing the ball – there it was!

This is dangerous! But it refers to all balls that include squeakers. So this is a general issue and not an issue of the Kong balls alone!

Left two images: Caution! The tiny squeaker easily pops out. Right image: Eventually the ball tears open.

One funny fact is that once the squeaker is removed they float much better. This is probably because through the squeaker when compressed they suck in water filling them. The squeaker blocks the way out of the ball for the water and traps it. So the ball just fills to the point where it sinks.
Something similar also happens with air. Once the ball gets compressed  the squeaker seems to suspend the air, preventing the compressed ball from expanding again.
This does not work any longer once the squeaker is removed from the ball. It still fills with air and water, but neither of them get blocked inside or supended.

Due to my experience with these balls I will remove all squeakers and only use the ones I have left until the package is gone. But I will probably not buy them again.

So, tennis balls for dogs yay or nay?

Well, a lot of dogs simply love tennis balls! And if you are only playing fetch with the ball, they may be a reasonable choice. Therefore I can only say so much:
IF you want to use tennis balls, make sure your dog does not chew on them and take them away as soon as the balls start to crack, fall apart or your dog starts chewing excessively. Make your own evaluation of the risk of your dog inhaling fibers from the ball. Never leave your dog alone with tennis balls or smaller balls (sizes they are able to swallow or choke on) in general!

Which kind of tennis ball you eventually choose will very likely be a matter of trial and error.

Find more advice on other choices of balls for dogs in this essay.