I've posted about trailers before, as have many others. I was curious about them so I bought a BOB Yak and took it from Seattle to Omak, Washington, over the North Cascades Highway last summer. Here's my review, in a nutshell:
Pros:
- Can carry a ton!
- Easy to pack - no need to worry much about balancing side to side like panniers, or exactly where everything goes. Just stuff it in the big yellow bag.
- Big yellow bag is waterproof. No need for garbage bags, pannier rain covers, etc.
- Handling is great. Others have posted to the contrary, but I didn't have any trouble with handling. In fact, after starting up after a rest I'd always check my rear view mirror to make sure it was attached!
- Easy to attach. Oh, there were those awkward moments when things weren't quite balanced and it would jackknife or get away from me somehow, but usually it was a simple matter to hook it on and get going.
- Great for bikes without brazeons.
- Diminishes chances for broken spokes.
- No need to buy racks (or panniers).
Cons:
- Weight - the thing weighs something like 13 pounds unloaded. That's a lot for bike touring.
- Rolling resistance - you can't deny that a third wheel adds some.
- Have to carry an extra tube. My 700 tube just won't fit the 16" tire.
- Different valve. My BOB wheel came with Schrader valves. I have a Zefal HPX pump. When I'd pump up my tires in the mornings I'd have to do the switch of the rubber thing on the pump head. Not a huge deal, but inconvenient.
- Harder to park and lock up. Again, a minor inconvenience.
My conclusion: Even though the trailer worked great and had some pros, I'm going back to panniers. The main thing was the weight/rolling resistance. On flats and gentle hills it wasn't bad, but climbing the passes was a pain! I started referring to it as "the anchor". I kept trying to shift to a lower gear and there wasn't one.
However, I'm still not convinced that I wouldn't have suffered similarly with panniers on that route - the North Cascades is a bear! I think the definitive test would be to something similar - steep and long - twice is fairly rapid succession - once with the trailer and once with panniers, so I could compare apples to apples.
For now I'm going back to panniers on my next tour, which probably won't be until next summer. We'll see then. Of course, it still won't be a scientific comparison because I bought a new bike - a Surly LHT, which should improve my experience with either rig.