I've used a Tubus, Jandd Extreme, and Blackburn Low-Rider. The Blackburn was fine and cheap. I think it flexed a bit though - no scientific evidence of this, just a feeling I have. It was on a flimsy Nashbar Touring bike from 1992 - XXL frame, 1-inch tubing....can you say, "Whippy?"

I put a Tubus on my LHT and liked it a lot. It was very solid, not too heavy, and the tube over the front made a great place to mount a headlight.

But I decided I wanted to try a platform. I got a deal on the Jandd so I bought it. The platform made a great place to bungee a couple of things, and the rack held my paniiers rock-solidly. It's probably the heaviest of the three, but not by that much. Downside - the headlight wouldn't mount on the front like on the Tubus. Oh well, I don't ride at night on tour and don't carry the headlight anyway.