Although I can't remember many of the specifics as it has been some time since I read it, I would have to say that Icebreaker is my favourite Gardner novel(and probably my favourite continuation novel overall). My experience of reading it was that it gripped me tighter than any other continuation novel. It has many of the elements that would reappear again and again in later entries (notably the double/triple crosses) but they are executed with aplomb and great pacing. The icy arctic setting is one of the more interesting locations as well.
I recall that in the James Bond Bedside Companion Raymond Benson was quite critical of Icebreaker and called it a significant step down from the first two novels, and perhaps if I was to reread the novel some of the cracks would start to appear. But I can honestly say that I liked it more than a number of the weaker Ian Fleming titles.