Still cheaper than a studio flash system

I work part-time as a photographer and have my system flashes initially ignited with the integrated transmitter of the 60D. This has always bothered me that the internal flash has to ignite as the master. After switching to full-frame (6D), a separate transmitter was necessary. With the ST-E2 I am what the function and operation concerns completely satisfied, however, I have my bought used, the original price is really crisp. The function itself, I would like a few rschreiben (for there is the Canon side), only about my personal experiences:
It is advisable to buy a spare battery, otherwise you stand in etnscheidenden Moment On Location and the juice is out! Here also the same my Hauoptkritikpunkt: The fact that an almost exotic special battery is required. When successor AA batteries are finally laid see. If the battery is already rather ershöpft it comes to annoying failures of transmission at a 30 ° angle to the left and right of the receiver. When fully charged, everything works flawlessly.
Control of non-Canon flash (eg Metz) without any problems. That not controlling the flashes in manual mode is a pity only possible as there may be fluctuations in the exposure depending on the measurement.
In summary: good equipment, but expensive!