seeing more and more people riding with these on steep rocky stuff but have a question!

Can you set the posts so it always drops to a certain level, and returns to the original height? or is it sort of just keep adjusting it again until its comfy? Sorry if this is a stupid Q but not sure how they fit / work!

as i said seen more and more people riding with these (still a minority though) but i wonder if one day they will become the pretty much the norm as front sus / rear sus / clipless has

Also whats the weight penalty (roughly) over a normal post 100g, 200g, more?

The post always returns (going up) to the same height, so you simply play about with where your ideal proper pedalling position is when the seatpost is at maxiumum extension, then leave it until you feel the need to drop the post further (into the frame)
There is no way of setting a minimum drop, but you get the hang of setting it by sitting on it. Theres enough resistance to do so.

It depends what you class as a normal post? Normal as in what came with your (budget) bike? A carbon post? A standard Thomson?
KS i900 I think was supposed to be 550g. The remote kit will add a touch more
A i950 is supposed to be a little lighter

A 3" drop crank brothers Joplin is about 460g, but is only 75mm drop

Are they up to taking some knocks?

I've fell off a good number of times? It still works?

The original lever for my i900 has broken twice (And no I don't run the lever exposed on top of the handlebar, but out the way next to the shifter lever. The lever is just brittle. The newer ODI compatible one shouldn't break as easily

After 9 months it does have a bit of play side-to-side play in it, but isn't noticable when riding. I think superstar can do some kind of service where the play is gotten rid of

The KS ones are pretty reliable, however UK grit and mud may have some effect.

I have the 900, a little weighty but an excellent bit of kit, one of those tings that you dont need until you try one.

Also get the remote