Yes, it will fire multiple flashes, in two groups with a ratio control between the two groups. It's not a bad option, however there is a distance limitation of about 40 feet indoors and even less outdoors.
Yes. The ST-E2 is flash tube with a red filter over it which cuts most, but not all, the visible light leaving the IR wavelengths. The idea is to make the command pre-flashes invisible to prevent blinking, which I've never found to be a problem using a 580ex master instead. Its one of the earliest parts of the Canon wireless system and like the first flash the 420ex it lacks the capability to remotely control EX flashes in M mode, which more consistent than ETTL in situations like portraits where the subject - flash distance (and exposure) are constant.
Its Achilles Heel is it low power and limited range, and it cripples the entire Canon system. It has nothing to do with IR, its simply a wimpy underpowered unit. Line of sight isn't needed to fire the slaves, but you can spit further than the ST-E2 outputs its light. Knowing this from research before switching to Canon I avoided the ST-E2 entirely and instead use a pair of 580ex in Master / Slave configuration.
To get the best possible performance always aim the base of your 430exII flash back towards the camera so it can see the light ST-E2, then rotate the flash head towards your subject and don't block it with a modifier. Outdoors the range is even more limited than indoors and you need to keep direct sunlight off the sensor on the slave.
WIth one flash moving it off camera trades flat light for directional light with harsh unfilled shadows. A better strategy for flattering light with one light is to simply raise it over the camera with an OC-E3 cord and a bracket. To move the flash off camera and control both pattern and lighting ratio you need two flashes: Master / FIll to lift the shadows overlapped by the off camera slave to create the highlights over the fill. Overlapping the key light over the fill is now contrast is controlled. The more fill there is relative to the key light the lighter the shadow get and the softer the lighting looks. My tutorials explain this: LINK
The 580ex as Master has more power and greater range and I haven't found it necessary to use radio triggers. So if you are thinking about expanding your system my advice would be sell the ST-E2, buy a used 580ex and use it and your 430exII in Master /Slave mode, then get the radio triggers if you encounter signaling range problems with the 580ex master.
Chuck is right when saying that the ST-E2 has limitations but ... I would say it has also some nice advantages: it is very easy to operate, it is lightweight, it can be used as focus assist, its indoors range is not bad at all (as Savas and Chuck said it does not need line of sight because the signal is bouncing around). I like using the ST-E2.
For outdoors I would like to get radio poppers but they are not adapted to european frequencies and I think these are not an option for beginners (basically because they are very expensive).
Claude
