Scott McCartney has a nice summary on the Wall Street Journal of the pros and cons of the airline alliances. Gary Leff‘s comments on the article focus on United’s starnet blocking.
Translating the global coverage of the alliances into useability for mileage earning and redemption takes care, and to be fair is a big enough topic on it’s own let alone in a summary such as this. Scott touches on some aspects such as the current restrictions on American and British Airways frequent flyer members for flights between USA and UK, the number of destinations served, and the influence the location of primary hubs has on travel useability.
Scott mentions one place where Oneworld is lacking coverage - China. I think there are other significant holes in the Oneworld network, for example Africa.
The key advice is that choice of airline alliance is relatively important for most frequent flyers, and the single biggest factor in the choice should be based on where you travel (or want to travel).