A few answers:

Weight

The BOB weights 13 lb (from another post);
The Burley Nomad (2 wheel trailer) weights 14 lb
The Charriot Classic (2 children trailer which I have) weights 22.5 lb and is bulkier (much more wind resistance)

Panniers add weight too:

- typical racks are about 1 lb each (total 2 - 2.5 lb)
- Arkel's T-42 and GT-54 panniers (which I have) weight 4.4 and 5.5 lb respectively.
Total for a complete set would be 12 lb.

Many trailer users like the fact everything is in one big pouch. But after a while -- and especially after a rainy tour --, they suggest adding a set of front panniers, so the stuff can be grabbed during the day without opening the big pouch. On some bikes, front panniers also help stabilize the direction.


On the road

I often tow the 2-children trailer and I have toured extensively with panniers. Except for those who ride singletrack, the trailer isn't much wider than my space, and a single-child trailer or a cargo trailer such as the Burley Nomad isn't wider than the width I need to sneak throug a narrow passage. There are a few places where it is a problem:

- on a highway with a gravel shoulder, the bike needs to be 12-15" away from the white line, otherwise the trailer rides partly in the gravel;
- on pothole-infested streets, it's harder to find a suitable place to ride and keep all thee wheels on a good surface;
- in the snow, three tracks really slow you down;
- in tight turns, the trailer tracks inside the bike; this is not a problem on highways and streets, but on bike trails, especially where they like to install chicanes ()
P.S. I find the latter especially bad when I tow a trailercycle and a child trailer.

BTW, most of these "problems" don't exist with a single-wheel trailer like the BOB, because the single wheel tracks behind the bike wheel and the hitch is designed so that the trailer doesn't track inside on turns.

Serious tourers use panniers

Or do they? Tradition helps here. I'm only 44, but I started to tour long before trailers were available. And if I tour with both children on a tandem + trailercycle, I'm not sure I want to add a trailer behind that. And given the price cargo trailers are sold here, I can buy 2 sets of Arkel's TT-82 panniers (they are huge) for the price of a good cargo trailer!

Apart from that, one piece of equipment that serious tourers have is a touring bike. I can attest that a real touring bike makes a difference. I have a 1980 Vélo Sport Alpin, which was defined as a touring bike then. Compared to my 2000 Trek 520, is is flexible and vibrates if I have more than 40-50 lb on the rear rack. And if I add too much on the front rack, it wants to control steering!

By comparison, the Trek 520 is neutral. I can load it as much as I want -- and I once carried 80 kg of groceries in 4 panniers and on top of the rear rack -- and it is as easy to control as an empty bike. So the only advantage of a trailer is for bulky items. BTW, I generally do the grocery with the trailer, because it's so easier to pack.

Should one use a trailer?

It depends. People with road bikes like the idea of a trailer, but their frame might be too flexible for a single-wheel trailer. People with a mountain bike, however, are well served with a trailer, because their bike is very rigid and allows a single-wheel trailer to track very well. See the warning about the BOB here: http://www.bobtrailers.com/warning.html


Other pros and cons (apart from what was highlighted in other posts

- The extra wheel(s) mean you have to carry extra spares. Parts for 20'" wheels are fairly easy to find, but parts for 16" wheels are not as easy to find in small towns.

- Extra wheel(s) mean less weight on the bike wheels. On soft surfaces, it may help.

- Torsional efforts (?) on the frame are induced by the BOB trailer and other single-wheel trailer. I have read more than one story of group tours where the only ones that broke spokes were people towing a single-wheel trailer.

- In a tour, it's easy to get rid of the trailer if you want to do a little ride after you have set up camp. But what if you need to bring a few items with you? A trailer and front panniers could be an ideal combo: use the panniers only for that quick ride. BTW, it's easy to attach a loaded two-wheel trailer like the Chariot or the Burley (with alternative hitch), but it doesn't seem easy to attach the BOB when it is fully loaded.