J.D. Power's 2006 Residential Online Service Customer Satisfaction Study contains a ranking of the leading IM service providers.
Rankings are by Satisfaction, Security, Performance and Reliability and Features.
Although voice components and feature sets are not part of this ranking, I'd have to think that voice functionality and quality did play a part in this.
Telling that Google Talk ranked so low. Google hasn't decided how zealously they should play this game. And Google Talk didn't even earn enough votes for reliability.