This is the second "Mammoth" collection I've read, and they've made me much more positive about anthologies in general. This particular set of stories includes some impressive names and even more impressive tales. I enjoyed almost every story in here. One or two were a little bit off, but most hit the five star rating with space to spare, which is way above the majority of anthologies that I've come across.
Alternate History stories often tend to fall into the same broad strokes, and indeed you'll find several long lived Roman Empires, histories where different religions gain ascendancy in different parts of the world and of course a couple of visions of how Hitler could have elsewise spent his time.
I don't really want to do a synopsis for each story as some of them are difficult to introduce without being either so bland that several of them sound the same, or going into too much detail and giving away the entire story. That said, a few hightlights of this collection for me were;
A Very British History by Paul McAuley - A look back at a very different space race.
The Lucky Strike by Kim Stanley Robinson - The most thought provoking story, about a different plane being assigned the Hiroshima bombing run, and the comparrison to a firing squad at the end is particularly striking.
and Darwin Anathema by Stephen Baxter - In 2009 a much more powerful Catholic Church puts the bones of Charles Darwin on trial for heresy.