Serious photographers looking to dip their toes into filmmaking will find the Sony a7S a very attractive proposition, though those looking for total portability will be disappointed by the need for an external drive to record in 4K.
The body of the a7S will be familiar to anyone who's handled the a7, as Steve Huff Photo notes "the A7s is just like the A7 and A7r in body, build, and feel. Controls are the same and LCD and EVF are the same." Photography Blog says it's "still quite small and slender," but TechRadar finds it "robust and solid in the hand." Photography Blog does note it's "a little quicker than the A7 camera," at least, though it "still lags behind the very best contrast-based and DSLR phase-detection systems."
All of the differences from the a7 and a7R lie in the A7S's image quality, and Steve Huff Photo says it "simply rocks and is in its element in low light scenarios," while Photography Blog notes it produces "images of outstanding quality." But the biggest selling point for the a7S is the ability to record in 4K, a selling point hampered by the need to attach an external drive to the camera to save the video files. However, Steve Huff Photo says that "shooting normal video is fantastic" and Gizmodo finds "the 1080p is really nice; better than most competitors."
Users looking for a 4K camera solution would be better served by the LUMIX GH4, which offers 4K recording without the need for an external recorder. But for most users, unless they have a 4K TV or plan to do a lot of low-light shooting, the a7 should suit their photography needs just fine for a much lower cost.