i have traveled on my aluminum fisher mtb xtracycle; it rules. yeah, it's a bit heavy, but i'm not racing and i'm a fairly strong rider anyways, so i don't care. it can carry way too much stuff - way more than you need for traveling unless you are someone who wants to travel with a guitar, surfboard, golf clubs, and television. i don't travel on my xtracycle because i want to carry a ton of crap - i travel on my xtracycle because it's one of my favorite bikes to ride. the long wheelbase lets you bomb downhill fearlessly - 50 mph seems like 30, but you may have issues with getting your bike on a bus rack or into a train stall. you can check out a couple of my xtracycle trips on crazyguy if you care (jabantik 2010 and 2011.)

longtail cargo bikes are freaking awesome grocery getters and commuters. if weight or speed are a concern, take a different bike

edit: my bike weighs 41 lbs. aluminum fisher mtb small frame, steel fork, xtracycle with the old freeloader bags, xtracycle kickstand, fenders, u-lock, and whatever else might be inside the bags - maybe a couple tubes, pump, levers, and a 4-5-6. it is heavy, but it still rolls well. sometimes i catch someone softpedalling and sit on their wheel - they will flog themselves to try to drop the guy with the big circus bike and bags of groceries.