I'm new to using multiple flashes, and really off camera flash in general.
I picked up a second 600 EX-RT for use with my 5D III. I'd like to play around with some studio type work have a couple tripods for the flashes, and would like some sort of softbox attachments (if they work).
Which would be the best for me? I don't really have the space (city living) to purchase umbrellas and such, are these little flash attachements any good?

Although I urge you to look at the size of the umbrellas in those kits. Often they come with 33 inch umbrellas, spend a bit more and get something in the 40ish inch range.
