So yeah I just used the 6th stim (witch was a Super Stim) on one of my party members (Linda I think) and as far as I can tell I can't see any difference in her stats. Also might be interesting to know what "side effects" people experienced. -Edit1- Its kinda lame it just subtracted 1 from another stat and gave me +2 HP .
That is not 100% correct, if you have used more then 5 stims the there is a chance that you get an random negative effect on any attribute. With every additional stim you use after that gets a higher chance to give a negative effect until it is 100%. Sometimes it also can happen that nothing happens as the effect just neutralizes the positive effect. It is some sort of gamble what you get with 5+ stims
I get the fact that if you are unlucky you can get +1 from the stim and then -1 (5+ Stim consumption) at the same time so "nothing happens" , but the weird thing was that the super Stim gives +1 to all stats witch means that I should have gotten +1 to at least two of the stats.