It is very good and I recommend it if one is keen on macro.
It enables working swiftly without a tripod using low apertures as f/13 with high shutter speeds.
It also has separate exposure settings for left and right lambs which can give better results.
Fill-in flash alone is very useful, can also help catching flying insects.
One thing to keep in mind is with low aperture and high speed settings background will be darker and more un-natural, which may be not desired sometimes.
I used a L-bracket with off flash, it gives good results but hard to work with and tiring. It is also very difficult or not possible to supply enough light at minumum focus range.
Some samples with Sigma EM-140:
here is a friend of mine who also uses this flash, he has a great set of macro shot here where that flash used -mostly the ones with darker backgrounds-
dragonflies - a set on Flickr