I recently made full use of the insurance I purchased through World Nomads--I was evacuated by helicopter while trekking in Nepal and spent six days in the hospital in Kathmandu. I had a mixed experience, and though I cannot recommend them, I don't know that any other companies are better. The actual servicer for our plan was Access America; I believe World Nomads has recently changed carriers, and who you are insured with may depend on where you live. Keep in mind that it won't be World Nomads you're talking to if you have a claim, so the knowledge and customer service of the actual insurance carrier will determine your experience. Definitely do your research and read the fine print carefully.
Bad experiences with World Nomads: We were unable to make an international collect call from Nepal, and had to use a satellite phone at about $1.25/minute. The first time we called, after going through a phone tree, we were transferred to AAA roadside assistance (WTF?). The second time, we were told they had no record of us in their system (double WTF?). Finally, after being put on hold for 20 minutes, we were connected to a nurse whose best advice was "go to a doctor" (we were four days away from the nearest city at that point).
A couple of days later, after I miraculously did find a Nepalese doctor who told me I needed an evacuation, we were unable to make an international call to the insurance company (not their fault). We had to contact the U.S. Embassy in Kathmandu, who took our insurance info and contacted the company to coordinate the evac. Numerous phone calls were required to organize the evac (luckily the owner of the tea house where we were staying allowed us to receive calls on her cell phone, though service was spotty). The agents we were working with clearly did not have a clue about where we were, since they kept asking us to fax them stuff (this is a TRAVEL insurance company, right?), and even the Embassy commented that they didn't get it. After about 24 hours, they were finally able to arrange a helicopter (OK in my case, but an unacceptable delay in a life-threatening situation). Once I was at the hospital, we also had problems getting them to authorize payment to the hospital in a timely manner. In fact, I basically spent an extra day in the hospital just waiting for them to respond so that the hospital would discharge me (in the end, my husband had to leave his passport as collateral and go back the next day to run interference between the insurance company and the hospital in order to get my bill paid).
Since getting home, we have filed a claim for additional medical expenses we opted to pay out of pocket rather than waiting ages for them to approve it, as well as claims made under the trip interruption clause. We have not heard anything back yet.
Good experiences: They paid for the helicopter and my hospital bill up front (good thing, cause the total ran to more than $60,000). I worry they may try to get something off our home medical insurance, who will turn around and ask us to pay a portion, but so far that has not happened.
If you will be traveling to remote areas in a developing country, especially Nepal, you might be interested in the extensive advice we compiled on our blog about what to bring and do in case of medical emergencies.