LoadImpact became instrumental for me in the early stages of projects where performance issues began to harm Net Promoter Scores and (worse) began to interfere with my ability to conduct qualitative research on the applications.
It seems like many UX tools suffer from similar problems: too much emphasis on slick transitions and UI candy, not enough emphasis on clear definitions and well-organized information. Sometimes, simplicity for simplicities’ sake isn’t helpful.
While LoadImpact does suffer from a lack of clarity for a lot of its functionality, it does a great job of deploying virtual users onto an application quickly and providing feedback about the performance of the site as you load users on.
Plus, it provides most of the tools (for small loads, up to 50 VUs) for free for occasional uses.