I use the same trailer Bike Friday uses (
http://www.bikerev.com/). I bought a reconditioned trailer for $100 on eBay. I then just drilled the holes in my Samsonite Oyster, and viola! You have to make sure to use the right frame attachment clip. For my Birdy, the C-clamp works well.
This is a very high quality trailer. The wheels roll better than my Chris King hubs. They contain two sealed cartridges and spin for quite a while.
You know the trailer is back there, especially when it is full. I was carrying about 50 pounds on my 10 mile commute to work today into a headwind and was fine, though it cost me about 20% of my journey to work time. On the way home, I was back up to normal with a nice tailwind and an empty suitcase.
There are two problems. First, you must mount it horizontally. Thus, you really need an orange flag if you want cars to see it. It would be a mess if a car bumped it because it's firmly attached to your frame. Second, you can't squeeze through tight spaces as well.
However, it was fine with potholes and rough surfaces. The tires a Kenda 40PSI. I love it.