When I tried St. Johns Wort, I had the same chemically induced anxiety/panic type feelings for the first month or so, and then they tapered down to my normal level. The reason I had to quit taking it was because it was over-stimulating me mentally, and I eventually couldn't sleep on it. Not anxious stimulation anymore but a kind of hyper feeling like I had taken way too much caffeine, without the caffeine jitters though, if that makes sense. This is coming from someone who used to be prescribed Concerta(an NDRI) for ADD, and I could always sleep fine once it wore off, and got no anxiety from it, except for an hour or two when it was wearing off.
I think maybe the fact that St. Johns Wort is a mild maoi contributed to the lack of sleep, as I didn't take care to monitor everything I eat to ensure it doesn't contain either tyramine or tyrosine...forget which one causes over stimulation if you eat too much foods containing it, while taking an anti-depressant with maoi properties.
Overall if it wasn't for the not sleeping thing(and I tried using more medical marijuana than usual to sleep, when usually a few tokes gets me to sleep), I probably would have stayed on it. It was really helping me not to care about my thoughts as much, and it's the only anti-depressant I've tried that works on serotonin which didn't cause sexual dysfunction as a side effect(maybe because it works on dopamine also).
Taper off of it a bit if you decide to stop taking it imo. I did a semi-rapid taper, and wasn't too concerned about discontinuation, but for a few weeks it made me feel even more depressed than usual :/