I found phenibut to be a bit dissapointing, but then i think i was expecting something stronger and more 'drug like' that what i got.
Phenibut seemed like very mild GBL to me. As in you got a similar (but much milder) sense or relaxationa and sociability without feeling as fucked.
However, because i was expecting more than this mild effect i tried larger doses, but these just made me feel nauseus and ill.
Phenibut can be addictive, especially if you take it every day.
I can't tell you much more than that i'm afraid.
Edit - except to say don't necessarily let my experience put you off. If you are looking for a very mild benzo alternative that wont fuck you up then it is possible you'll find phenibut more agreeable than i did, and it's not exactly expensive stuff.