Benefits and drawbacks of Dropper posts? From what I've seen, the last couple of years saw a new product type come into light, and since its inception it has started to appear on bikes (such as this one, this one, or this one) as a stock item. 

I'm talking about the "drop seatpost". 

For the un-initiated, these are seatposts that are adjustable "on-the-fly", either by remote or a lever under the sadle. The idea is to allow the rider to find an optimal seat height to suit the conditions. This seems like a great idea, but what suffers as a result? I believe there is an inevitable weight penalty that would burden the gram-conscious amongst us, but what else might sway a decision for or against the purchase of one of these items?

I assume the potential problems will include weight, mechanical failure and cost, but what else might there be to the equation?

From what I have gathered these nifty looking contraptions come in a plethora of variations, and notable brands have created their own versions. I've seen them made by Fox, Rockshox, Crank Brothers, Specialized, KS
and Gravity Dropper. Perhaps there are others as well.

So in addition, what differentiates each of these models and brands? What are the pros and cons of each? Is there one that stands out as a clear winner in the field?

Any help or insight is warmly welcomed, so thank you in advance!
There are generally two types of dropper seatposts, mechanical (e.g. GravityDropper) and hydraulic (e.g. RockShox Reverb). Mechanical dropper seatposts use a spring to move the seatpost and a bolt to keep it in place. This is a very simple design and there are few things that can break or jam, and the weight is also kept very reasonable since there are few parts needed to make it work. The drawbacks are that you only have a fixed number of height settings, i.e. fully extended, halfway extended, and fully lowered. This is because the seatpost is being held in place by a pin that goes through a hole in the seatpost.

A hydraulic seatpost on the other hand uses hydraulics to move the seatpost and thus allows you to place the seatpost at any height you like. The drawbacks are that they are more complicated, usually weigh more, costs a lot more and requires more maintenance. The advantages are the ability to lock the post at any height you like.

Mechanical dropper posts:

 + Mechanically simple and easy to maintain
 + Low weight
 + Cheap
 - Only a fixed number of positions for height adjustment


Hydraulic dropper posts:

+ Freely adjustable to any height
- Complicated
- Requires more maintenance
- Expensive


Static QR seat posts:

+ Requires no maintenance
+ Cheap
+ Low weight
+ Can be adjusted to any height
- Rider must dismount to lower and raise seat