Others will probably weigh in but I'd say it depends:

- if you live near a member airline's hub, maybe a B-
- if not, particularly in the areas where the alliance has poor coverage (see cons), then probably a D-

Pros
1. Good transatlantic coverage
2. Ability to use miles to upgrade on partner airlines (but see con 1.)

Cons
1. Ability to use miles to upgrade on partner airlines restricted to tickets purchased in Y class
2. Absence of alliance wide luggage benefits for elites
3. Least useful of the three alliance websites (e.g. poor/hard to find timetables, no online round the world booking ability)
4. Lack of South American and South Pacific members, with poor coverage of the southern hemisphere and South East Asia
5. Maybe perceived to be the slowest growing of the alliances