If you consider exponential progress since 1970 or so to be "little success," you're right. Fusion has a very high threshold before it becomes useful, but we've come a long way, and we're not that far from the breakeven point now.

NASA is currently working with John Slough's company on a fusion rocket for interplanetary travel.

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...

http://www.washington.edu/news/2013/04/04/rocket-powered-by-...