When I was a student living in London a few years ago, I didn't bring any traveler's checks. Just completely forgot.

I can't speak to the cost-efficiency of these options, but as far as convenience, I used my credit card and ATM the entire time, even traveling to the Continent. In Paris I found a few ATMs that wouldn't take my card (not on the right network) but always eventually found one that would work.

One of the biggest advantages of traveler's checks is that they seem relatively easy to replace. I had a friend whose checks were stolen, and he just went to the nearest Amex office and got replacements. Total time spent: about 1 hour. On the other hand, if you lose your credit or ATM card, I would imagine they're not nearly as easy to replace.