I don't have any hard data on tag clouds specifically. Although, like Matt mentions, I find the tag list with count on this site's search pages to be more useful.
But the problem remains that in a community-driven website, tags in general often scatter beyond their point of usefulness. In other words, when tags are generated by members, you'll often end up with two tags that are for all intents and purposes synonymous. But by searching the two synonymous tags you'll see very different results.
For this reason, it's often beneficial to also segment your site into tightly controlled categories. Obviously this will depend on the nature of your site. But if you design your categories to have a clear separation and no overlap, then that can be very useful. The navigation-by-category UI should then be above the tag list/tag cloud as it's likely to be used much more often.