I have always had one of those adjustable tiller extensions until I got the Blade, which has a fixed length fiberglass rod. I now find I like it much better, it never slides in or out when you are steering and I don't ever have to play with it while racing, just flip it around the blocks, grab it and go.

I think most of the A cat guys use a fixed carbon rod but at about $300 each, a little too much for me. The fixed fiberglass rod is less than $20 but quite a bit heavier. I don't think it slows me down though. A compromise might be a fixed length aluminum tube, but they will bend (or break) if you abuse them! I doubt I will ever go back to one of the sliding, adjustable types.