you will get gifts for every 3 you save. it doesnt matter how many you harvest. but yeah you wont be getting the achievement, its worth like 100 pts if i remember right.

quote Shadow net583x s

The Adam really comes in handy, and I dont know what Tenenbaum gives you for not harvesting them.

you always get 200 ADAM in the gift, plus some other stuff. sometimes its a Tonic, which you can always use, especially if you plan on getting the Tonic Collector achievement.

playing on Hard, i rescued ALL the Sisters, and i had plenty of ADAM. its true in the first part of the game, you are always a bit low on ADAM, cause there are always good things to buy in Gardens, but about 3/4 of the way through, i started amassing an assload of ADAM, and at the end of the game, i believe i had quite a bit left over after buying like, all the Tonics i could buy, plus all the HP and EVE upgrades you could. i didnt however, buy several of the Plasmids, but they were ones i didnt really care about having, and there arent any achievements tied to buying all the Plasmids. but i bought the 3rd versions of both the Electricity and Fire Plasmids which is all i really cared about buying, TBH.