I haven't used Blick's, but have used other pre-stretched canvases as a source of inexpensive stretchers. Not being in a metropolitan area, and with slower and more expensive shipping options available than most places, sometimes it is my only option. The ones that are sold here are made with lauan mahogany, a very lightweight tropical hardwood, which is not quite as nice as the kiln dried sugar pine that is more common for unstretched bars. That said, I haven't had any issues, other than an occasional divot on an edge or corner. It is somewhat sad that it is substantially cheaper to buy canvas and stretcher and get rid of the canvas than it is to buy the wood only, but a reflection of the fact that the ones I have available are made in Vietnam, vs domestic plain bars.
You might want to take a look at
http://www.aswexpress.com/, if you are still shopping around. They are the absolute cheapest online source I have found for online art supplies, and have decent customer service and a good selection of stretchers..