I am a bit of a gadget junkie, I admit it. There are plenty of art tools I own that my wife will be happy to point out to you, were a "waste of money," as they just sit in my studio, never getting used. (alright, that's a bit unfair; she's actually quiet on my purchases, which is probably worse, since I just keep buying more then!). I have tried different brands on many different items, and have often learned the hard way, which item was the best purchase. Therefore, I will occasionally post my picks, so that a person reading this down the road won't waste their money on an inferior item.
If you are planning on stretching your own canvases, and are in need of canvas pliers, then the only choice is Holbein. Their model HK 1053-5 canvas pliers are the only ones I've tried that can actually grip canvas and allow me to bear down and really stretch the fabric tightly. They list for $145 USD, but if you shop around, you can find them for nearly half that.
I've tried many other models that ranged from $10 to $60, and they were all horrible. I should have just spent the money in the beginning to buy the quality pair pictured here. Don't be fooled, however; there is another company out there which makes a generic pair that looks just like this, but the don't compare. The knockoffs are sold under various names by the big catalog companies, so make sure that what you are buying is specifically manufactured by Holbein.