This sixteen part series is a spin off from the 'Haruhi Suzumiya' franchise; it takes place in the alternate reality introduced in 'The Disappearance of Haruhi Suzumiya'. In this reality there aren't aliens, time travellers and ESPers, as much as Haruhi wishes there were. While it isn't strictly necessary to have watched those series and film to enjoy this, I'd advise watching them first as many events refer back to them.
Centred on Kyon and Yuki, who here is a rather shy girl, two members of their school's Literature Club whose lives are somewhat changed when they meet Haruhi; who soon establishes herself as a leading member of the club despite attending a different school. It is clear from early on that Yuki has feelings for Kyon and as the series progresses it seems he is developing feelings for her although neither will admit it to the other. Then an accident affects Yuki in a way that could change her forever and her friends must cope with this and help her.
I can understand how some viewers might be disappointed as this is quite different to 'Haruhi Suzumiya' I didn't have a problem with this although I won't deny that I thought the series picked up when Haruhi was introduced. The early part of the series provides plenty of laughs and creates a feel-good atmosphere which is shattered when Yuki has an accident. This leads to some tear-jerking moments before things are resolved. Yuki is a very likeable co-protagonist and Haruhi provides plenty of laughs. Kyon, the only character who is effectively unchanged, is a solid 'ordinary' character. The rest of the cast are fun too although some previously important characters now take a surprisingly small role. The series does contain a little more fan service than the original, especially in the hot spring episode, but it is never excessive. Overall I'd definitely recommend this to fans of the franchise.
These comments are based on watching the series in Japanese with English subtitles.