I was glad I knew going it that it was to be continued, because otherwise I would have wailed at the end.
I found the multiple viewpoints a bit confusing and disjointing at first, but after a while, I got used to it and enjoyed seeing what was going on with each person. I admit that a couple of times I skipped over a chapter or two to continue a specific person's story and find out the resolution to a mini-cliffhanger. I did find I was more interested in Polly and Merope/Eileen's stories than I was Mike's. Perhaps because I'm traditionally more interested in what was going on on the "home front" with the Blitz than the actual fighting and military stuff.
I had never heard about the inflatable-tank decoys. Funny!!
Since we never heard anything else from Mary after she got caught outside in the bombing, I figured that sadly meant she had died. Did others draw the same conclusion?
My guess for the unnamed time traveler at the end is Colin. I wonder what's going on in Oxford to make all the drops fail, but yet allow someone else through? I rather wonder if it's deliberate on Dunworthy/Oxford's part -- the rampant drop-switching was extremely suspicious. Perhaps they're trying to disprove the time-travel opponent's theory, though that strikes me as dangerous and I can't believe Dunworthy would endanger the time-travelers like that. Theories?