Rhodiola noticeably lowers Cortisol (stress hormone) so it becomes much more harder for anything or anyone to stress you out. It boosts Dopamine, that directly increases mood and positivity, increases confidence and sociability. If you're a runner or cyclist it boosts efficiency of O2 uptake so you could do silly things like hold your breath while running. If you feel sad in bad weather - this could be compared to a mild Antidepressant: it might be drizzlin' outside and the sky is grey, but you will feel like signing in the shower. Absolutely fantastic plant.
If you want to try out a new supplement, start with AOR: it's medical grade, meaning every single pill will have exact amount of active ingredient in it, it's tested to ensure that and it also means 99.9% purity. Their website has PDF files on medical trials, history of use and are all you need to know about this plant.
This brand is usually more expensive, but it's top quality with guaranteed results. After you know that a new supplement works for you and you know your dose, you can always move to cheaper alternative. However, after trying 8 different Rhodiola products over the span of six years, I believe this one to be the best. AOR Rhodiola + Ginseng is also good, but if you're a man - Ginseng will make you horny (try with caution ; ) and it has stronger punch in physical endurance department. AOR Rhodiola + Ashwagandha is good too, but Ashwagandha is a little too calming for me, just a touch, this one is good for recovery and as a tonic.