very good unit and quite capable

This flash is a ring flash which is usually used for macro photography.  It isn't the most powerful flash but you don't need the big numbers when the subject is only 30 cm away.

I won't bore you with stats - you can look them up.  What I'll concentrate on is usuability and performance.

The reason why I went with this flash over the Canon twin head macro flash had a lot to do with price.  This flash is much less than the Canon flash.  I've always been very happy with Sigma equipment and for a flash that doesn't get as much of a use and others I'll save the cash.  That said I have to emphasize that this is a very good flash.

The Sigma flash is extremely capable, it has all the modes E-TTL (A & B), manual, TTL etc.  You can ajust each bulb separately or just set it to auto and enjoy.  The flash will also work as a master to other Canon or Sigma flashes set as slave.

The manually is pretty thin, while everything is covered it's not covered with examples.  That makes some of the advanced concepts difficult to use.

The unit itself is well put together with the power unit on the hot shoe and a thick coiled wire connected to the flash head.  While on the camera the unit isn't too bulky - you can stow the combination in a top load bag if it's a larger one.  Usually I use this flash on my 100mm macro lens and the flash doesn't make the camera too front heavy.

Attachment of the flash head is via an adapter ring screwed onto the lens' filter threads.  The flash head will then attach to the adapter with two spring loaded tabs.  It is very easy to detach the flash head for lens changes or for storage.  One thing that is a pain is that its very hard to use the standard lens cap with the flash head on.  What you can do is use a larger lens cap and snap it in the head itself.

Overall I'm very happy with my flash, it's much less expensive then the Canon unit and Sigma has a better warranty.  It performs well and has lots of potential for more manual adjustments.