I used a BOB for 1200km in Baja Mexico. It was great, I would use it again, especially when we were going through the 200km stretch with no water. I was carrying over 20L of water (which is 40lbs), I also carried other gear. The others in my team carried some of the water as well, but I took all the heavy stuff because I could, and there were no problems as long as you are under the max weight (30kg or 70lbs). You do notice that it is a little bit more sluggish (especially up the hills, in which I had to walk up a few of them). I do think it is worth it though. The problem is when you want to go over 40km/hr down a hill, you are not supposed to because it is sketchy, very sketchy.
My girlfriend found it was convenient to have the BOB because I could carry more, I even took her front paniers and put them on my back rack. The BOB is for those that want to carry a lot and not worry about what it is doing to their bike. Being a gearhead, I always have a lot of gear the is not necessary, but having a BOB is great. In town, you can carry groceries a lot easier with a BOB. Traveling on plane with a BOB is great as well, just take off the front trailer fork and the wheel, protect the back fork and put your giant yellow bag in the trailer and put it on a plane as checked baggage, all other parts and wheels go into your bike box.
I'm happy, the BOB gives you options, I had 2 flats on the bob, but none on my bike (I am going to look for a Marathon XR type for the BOB). I want to check out the Xtracycle as well, that seems pretty cool and more options (extra person on the back, no hitch ect). Maybe not for touring though. The BOB follows you everywhere you go, and you do feel it behind you, you get used to it and it handles great.
Really at the end of the day I don't think it really matters what you use. Panniers are nice if you want to hop on a train or a bus, but it isn't a big problem to put a trailer on a bus (I have many times and there were no problems, in Mexico at least). That is my opinion and I am a really easy going guy that likes a challenge (like pounding up a steep hill with a trailer that weighs more then the rest of the your team, just imagine the size of you legs when you are done).