What are the pros and cons of different types of smart switches (that are on the US market) ?
X10


Pros:

Many manufacturers, and wide variety of products are readily-available (As of 2012, this may not be the case any more)
Low-cost

Cons:

Old protocol (circa 1975)
Most devices communicate over power lines, and design makes it susceptable to interference from noise. At worst, noise can be interpreted as commands (causing lights to turn on). 
Slow and fairly basic. Sending more than 2 or 3 commands takes a couple seconds. 
Only supports 256 addresses per network (total number of discrete dimmers, scenes, and sensors combined)
Requires phase-couplers to bridge both phases of power together
Requires PC or special hardware to program 

Products

Smarthome makes fairly decent quality switches, which support scenes (multiple devices responding to a single address). 



Insteon


Pros:

Relatively low cost compared to other products on market
Decent quality switches 
Basic linking (including scene programming) can be done using devices themselves, PC-based programming for more advanced control. 
Each device has unique address (16 million possible), and supports 
Hybrid powerline/wireless: many devices use both to communicate, and most devices repeat signals to increase coverage/signal strength
Easy to interface to other products due to market size:

There are even several iOS apps that can control Insteon if you have an ethernet bridge
Most home automation controllers can communicate


Cons:

Swapping out a device can be difficult due to addresses being hardcoded (need to reprogram all other devices that were controlled by or control the swapped device)
Some strange limitations in scene programming (eg, cannot program a scene that sets some devices on, and others off) 

Products

Smarthome makes a wide variety of products and is also the designer of Insteon



UPB


Pros:

More reliable protocol than X10
Simply-Automated's switches (at least) are extremely flexible:

Can configure 1,2,3-tap and press-and-hold behaviour for every button
8-button switches are configurable with interchangable faceplates to be anything from a basic dimmer to 4 dimmers or an 8-button keypad or almost anything in between


Cons:

Uses power-line carrier, so it requires phase-couplers to bridge both phases of power together
Complex to program, requires PC software

Products:

Simply-Automated makes switches, plug-in modules, in-line modules, I/O connectors and computer interfaces



Z-Wave


Pros:

Wireless, and each Z-wave unit acts as a repeater, so the more devices you add in your house, the better coverage you get.
Standardized and easy to D-I-Y, you don't have to have it professionally installed

Cons:

Still a technology 'maturing', so you might have to be a bit of a hobbyist to get it all to work correctly.

Products:

Thermostats by Trane and others
Light switches by GE and others
Locks by Yale, Kwikset, and others
Controllers by many vendors, either PC-based (Homeseer, z-wave.me) or separate devices (MiCasaVerde, Homeseer)