I've tried them. They're just barely okay, if you you're expectations are what they should be. Not intensely flavorful. And they are over-optimistic about how much stock they will make.
If you want to keep this sort of concentrated stock, try MoreThanGourmet. Their stocks are highly reduced actual stock. About the same size package. They also make demi-glaces that are equally genuine. And I keep their 1.5 ounce duck fat packets. Perfect for cooking for two, and duck fat is special. They also make 20X reduced veal stock, lamb stock, seafood, and venison stock that I haven't tried, but I would expect them to be good. And all their stuff is real reduced stock. I did not like their red wine sauce. Probably just a matter of personal taste, but I thought it too dark and heavy.
Classic French Stocks & Sauces | More Than Gourmet
Obviously, they're going to be more costly than Knorr, I buy off Amazon:
Amazon.com: More Than Gourmet: Grocery & Gourmet Food
For example, six pack of 1.5 ounce chicken stock reduction for $14.75. Sixteen ounce container Fond De Poulet Gold Chicken Stock for $22.92.