Are they worth it? Say if I get 4-6 seals(+.5%) that's 2-3% more health... how good is that compared to say health regen or mana regen? or 1.41(5-8 armor)

Armor is much better. Supposing you had Ahri, who has 71.5 armor at level 18. Increasing her armor by 5 increases her effective health by 7% (76.5/71.5=1.07) against attack damage, vs 2-3% for health seals.
Doing some calculations, the break even point is a natural 166.6 armor, after which it is better to get health seals. However, doing this will likely not benefit you, as it hurts your early game before you get that much armor, and doesn't make much of a difference afterwards.