They aren't miracle balls, but they last longer than tennis balls
My 100 pound Italian Spinone chews tennis balls the way 13 year old girls chew gum. Oh, he'll chase them halfheartedly, but once he gets to them he does some SERIOUS munching on them. The problem is, he really won't play with ANY balls except tennis balls (and I've tried a LOT of ball substitutions, believe me). I've literally seen him destroy a tennis ball in 10 minutes.
He likes these as well as "normal" tennis balls, but they will usually last us through several play sessions before he cracks them. And, even once they are cracked, the felt will sometimes take a couple of more days before it is destroyed, meaning that I can sometimes get a week out of one of these balls. Not great, but much better than the regular tennis balls.
If you are looking for a ball that cannot be destroyed, look further because this is NOT it. But, for our house it's a pretty good compromise.

Short lifespan
I don't find these balls "tough" at all. I purchased 20 packs when they were on sale for $1 as expendable balls for ground chase and river fetch at the local dog park. Instead of losing them, they're being expended by falling apart. In less than two weeks, my dog (not huge - a 45 lb. year-old lab mix) has split four of them during routine play. Penn tennis balls seem to withstand a lot more play. At the price paid, its tolerable, but I'm giving one star because I'd be really unhappy if I had paid full price.

Great for puppies
These work great for my 12 week old dachshund. Just the perfect size. The only down side is his sharp puppy teeth sometimes end up caught on the ball. After a few weeks they look bad. But that may be with any tennis ball.

4 out of 4 Paws
First Impression: Now let me start off by saying I'm a tennis ball aficionado. I can entertain myself for hours playing soccer, trowing the ball for myself, bouncing it off peoples feet so they don't even know they are playing, etc. But tennis balls tend to fray and Dad doesn't like it when I start pulling off the felt. Plus with enough time I can split a tennis ball wide open. So the wonderful folks over at Petsport came through again, and sent me some Tuff Balls! Word around the dog park is they have a 4 inch model too. I need to get my huge paws on one of those!

Conclusion: Tuff Balls are industrial strength, extra thick, non-toxic, and colorfast! If your puppy love tennis balls as much as I do, I would highly recommend trying Tuff Balls! I've been chewing on mine for days and except for the dog hair that's stuck on it, it's just like new.