I've been using microStock Solutions since they acquired my old stock agency over 3 years ago now.
It's by far one of their best products with regards to customer service & quality assurance as well - which in turn has helped us grow our business significantly (by more than 50%) from when we were working solely through StockXchange. The only downside really that i can think about would be if there was some sort of API available so you could integrate it into your own system but at this time, its not possible unfortunately unless otherwise stated! As previously mentioned though, all communication/support issues are dealt very efficiently via email or phone call and often times within minutes too rather then days like other providers have taken weeks before responding. With respect specifically to content creation services such as editing images / videos etc., these guys do an awesome job providing customisable layouts upon request along side any edits required.