Actually it's not 50% off everything over that time period. Essentially the way it works, is that if you buy products now at full price (not 50% off) they will email you a gift certificate in the amount of half your current purchase. That gift certificate is then redeemable during those dates.
Example, (I am making up the prices), Buy 2 bottle of 1 AD for $60 a bottle, total price of $120, you get a gift certificate emailed to you for $60, redeemable during those dates (sometime in March through April).
Overall, I think the Powerstore's prices are competetive, you can find cheaper, but you can definitely do worse. I still think it's a pretty good deal. I just ordered from there, and they did in fact email the gift certificate.
I did some quick math and compared my order, which totaled to $180 to another site. My order there would have been closer to $150. But I am getting a $90 gift certificate, granted I will be redeeming it on stuff that's a little higher priced, but the point here is you come out a decent amount ahead of the game.
I hope some of that made sense.....