The premise revolves around two alien beings, both shape-shifters but of a different variety, who've been on Earth for aeons and whose futures are interlocked. The protagonist alien's character develops with each page turn. The pace in which the story unfolds is gripping, so too is the action, and there is mounting excitement and tension as the decades pass and the two diametrically opposed mimic's paths intersect. (Highlander) tropes abound as both have embedded themselves into human history, making do with their special shape-shifting abilities.
All this was very cool.
Now, if it weren't for the central human character and his middle-age crisis story arc, and the 'tired and contrived' (Close Encounters of the Third Kind) ending, this could have been an outstanding work of SF. The evil alien antagonist wasn't helping either. Where there was scope to explore some genuine villainy, the character delved into the cliche world of Nazi bad guy strudel.
I enjoyed this read immensely but it remains for me a major 'if only' science fiction novel.
Was it worth the read? Yes, with a smidgen of disappointment.
Haldeman fans will forgive, others may not.