Heatmaps show you where users click and what attracts their attention, they don't show you WHY though. User testing, on the other hand, shows you WHY they get attracted, what they expect when they click, and what confuses them.
Userfeel allows you to find real users' problems and develop informed hypotheses in order to run more focused and targeted A/B tests. So, user testing precedes A/B testing.
Based on Nielsen Norman Group study, 5 users reveal about 80% of the problems. “Adding more users increases costs, yet the number of findings quicly reaches the point of diminishing returns”.