I love Kay's later works, Tigana, The Lions of Al-Rassan,
Sailing to Sarantium and
Lord of Emperors, so I was prepared to be forgiving of this earlier work. It is a competent enough portal fantasy, with five young Canadians wrenched into a largely Celtic world to fulfill a variety of quests. There is some odd pacing of info-dumping, and the characterisation is not as good as in Kay's later works - did he take the wrong lessons from his work on The Silmarillion? But it's decent enough, and is interesting for the way it draws on various different cultural roots without too much disharmony - the Summer Tree of the title being a particularly good example (Neil Gaiman uses it in American Gods as well, but more intrusively). So I will read the sequel in due course.