Semax is backed up by decades of studies and clinical use as being a safe and reliable peptide, despite its mechanism of action not yet being fully understood. Along with a low toxicity profile, side effects do seem relatively minimal (though a stimulant sensitivity could increase anxiety).

As for my own experience, I found how quickly tolerance increases to be the only issue I see with it, though that's only a problem with heavier use. Also while I personally don't mind it, I can see some disliking that it has to be taken as a nasal spray, or how it needs to be stored in the fridge (after opening) due to its sensitivity to heat.