Didn't get a chance to finish (telephone), but wanted to add:
>The ones I've had using the threaded rod are tricky to get square with the frame in it. Four knobs
to turn for squaring the frame.
>The one (I built mine from BB ply) that tighten in the center also tend to bow up when tightening.
I have a set of Bessys that I'm not crazy about either. I use pony corner clamps if the frame is not over 2" wide, then I have some corner blocks I cut out of plywood for the bigger stuff. I've been wanting to try the clampmate, from Rockler. http://www.rockler.com/product.cfm?page=725
Scott I have an older one very similar to the one Joe pictured that I use and love. Great for odd angles and other than 4 corners.
I have a ridgid frame one that is supposed to work the same way... not I said supposed to! argh!! its in a box to give away to the next woodworker I find I don't like! (just kidding)
My experiences match yours.The Bessey corner "clamps" are very useful for positioning pieces, like cabinet sides, for other operations like driving fasteners, but are really ineffective as clamps. I just reread their description and they avoid talking about using them for glue-up clamps. I use the Bessey variable angle strap clamps for picture frames and boxes and they work well for me. You can check out the details here.
Scott, Rockler sells quick release five star knobs in various thread pitches for about $3.50 - $3.75 each. Slide them up snug, and then tighten. Make your own corner brackets from hardwood, or plywood, with tee nuts in them. Tee nuts are very inexpensive from Reid Industries. They will sell you a bag of a hundred for about what you pay for a pack of 10 elsewhere.
I have a brand new Clampmate from Rockler that you are welcome to borrow if you want to try it.
I modified it slightly. I epoxied on some small pieces to the corner pieces to allow a relief for the corner of the frame. This way the piece actually puts pressure on the sides of the frame pieces.
I also saw a tip in a magazine once. If you cut some small triangular pieces of scrap wood, you can glue them to the sides of your frame at the corners. This will give you two clamping surfaces that are parallel for which any c-clamp will work. The trick was to use brown paper between the wood when you glue it. After the joint dries, you can (supposedly) easily separate the triangular pieces, then sand off the left-over paper. Never tried it, but thought it was a good idea for a tight joint.