I have been using a Topeak Beam Rack for several months now with one of the Topeak rack bags. It seems like carrying anything over 3 or 4 pounds will cause it to slowly shift. Not a lot but enough to bug me. I have it clamped down as far as it will possibly go. As mentioned earlier, you can buy the optional pannier holders but in my experience I wouldn't try it.
This week I ordered a Topeak Explorer rack because I need to carry more without the worry of shifting and want the option of panniers. The beam rack is going on the mountain bike. I really like the Topeak rail system and recommend it. Here's my beam rack: