The HTC Butterfly S has taken the best aspects of the HTC One and mixed them up with an amazingly long lasting battery to provide a truly all round device. The device features a 5.0 inch touch display, 16GB of internal storage which can be expanded through microSD, a 4 megapixel Ultrapixel camera capable of 1080p HD video capture, a secondary 2.1 megapixel camera for video calls and is powered by a 1.9GHz quad-core processor with 2GB of RAM.While you will definitely love the display and front stereo speakers, its sometimes unflatteringly large size and less than perfect photos may result in you looking away from this handset.
Its a little unclear why HTC chose to market the Butterfly S alongside the HTC One, considering their similarities. However its likely that HTC simply wanted to saturate the market and give their customers enough choices to ensure they don't select a different brand.
You can read a detailed video review of the HTC Butterfly S on PhoneArena.