tl;dr: Creatine reliably boosts strength by ~5-15% by acting as a rapid energy reservoir, and it's clearly important for brain function (deficiencies cause intellectual disability), but evidence that supplementation makes healthy people smarter is weak. The famous 2003 Rae et al. study showing huge IQ gains failed to replicate, and recent meta-analyses and the EFSA find no consistent cognitive benefit. If there's any effect, it's likely small (1-3 IQ points), possibly concentrated in vegetarians or stressed individuals.