Is that as good as it sounds?

4.5% with runes and 3% with masteries.

no

even if you could have 10% at the average base HP of 2000ish you only get 200~ or so extra, the scalling hp would still get you there.

you buy these runes if you actually plan to buy alot of hp, not good for tanks since they thrive of some hp and alot of resist which the runes for armour/MR are a much better investment.

It's pretty okay if you're going to run support shen, his heal is actually really strong this way.

hp/lvl runes are actually much better until late game, provided you're intending to stack health in the first place. I don't remember how much it was but you need somewhere around 3k health before the max hp % runes become better.

I personally don't see the point in using them because they take so long to become marginally better than existing options.

You can also get more info in this topic.

Do you play 3 warmogs mundo? If your answer is no, then my answer is no.