Alternate history is a fascinating genre and, when done right, a fantastic read. I agree with pdr in that I like my history "straight", but I think there's a game theory aspect to alternate history that can shed some valuable light on what really happened and how tenuous the links were that led to the history we know.
Honestly, I've never understood why alt. history gets stuffed into fantasy so often. If it involves time travel/aliens/etc, alright, I can see putting in it sci-fi, but the "organic" alternate histories strike me as more straight fiction.
As a reader, my take on alternate history is the same as historical fiction - I want it accurate, and I want it plausible. Most alt. histories hinge on a single point of divergence, and if that point isn't plausible, the whole story usually falls apart.
A few of my favorites:
How Few Remain - Harry Turtledove: This series got sillier and sillier as it went along, but HFR, tracing the story of a war between the USA and CSA in the 1880s, was in my opinion both accurate and plausible. The point of divergence was organic (soldiers finding Lee's dropped cigar case containing orders), avoided Antietam, and set the stage for a British intervention.
Gettysburg - Newt Gingrich: Yes, it's Newt, but this is alt. history at its finest. The point of divergence is Lee listening to Longstreet and marching in a wide flanking maneuver, rather than throwing men at Little Round Top.
The Fox and the Rhine (can't remember the authors): The Valkyrie plot succeeds, Hitler is assassinated, and Himmler becomes fuhrer. Rommel heals from his injuries and is given command against the Allies after D-Day. There are some outlandish elements (a massed Me-262 strike against a bomber fleet), but for the most part the story is well handled.
Rivers of War - Eric Flint: Sam Houston avoids injury at the Battle of Horseshoe Bend, with significant ramifications on the War of 1812.
I've also read my share of stinkers, mostly involving implausible divergence points or outright hero worship. One, in fact, is one of the things that motivated me to write my novel (figuring if it could get published, then my story stood a chance):
Hannibal's Children - John Maddox Roberts: The premise is promising - Hannibal defeats the Romans and banishes them across the Alps. A century or so later, they return, and all hell breaks loose. But the point of divergence is inaccurate and implausible. It involves Hannibal appearing outside Rome in the fall of 216 B.C., his army swollen to absurd size thanks to the support of Philip V of Macedon. The problems with this are many:
- Hannibal and Philip didn't strike an alliance until 215, and Philip never sent a single soldier to Italy in support of Hannibal
- Even if the diplomacy had occurred earlier, there is NO WAY Philip could have mobilized and dispatched an army of 200,000 to Italy within three months of the Battle of Cannae
- Quintus Fabius Maximus was not dictator after Cannae. That would be Marcus Junius Pera
- Ignoring the gross mishandling of Publius Cornelius Scipio's character, he was 19 at the time of Cannae. His rank as military tribune would have given him some authority on the battlefield, but none in the Senate (which he would not join for another three years). And yet Roberts has the tie-breaking vote on whether or not to accept Hannibal's terms falling to him.
I guess what I'm getting at is that alternate history, like historical fiction, can be done very well, or very poorly, and that just because it's alternate history doesn't mean one can avoid the historical reality at his or her leisure.
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