I really like the grit mechanic of the Gunslinger (the name for the mechanic is a little meh, though). I think it's a very strong mechanic because players are rewarded for taking chances and doing dynamic things.
Gunslinger looks very interesting and it definitely looks unique enough to be a base class.
I like the Ninja also. Rogues don't appeal to me very much, but the Ninja is something I would definitely play. It's just has much more flare than a Rogue - regardless of power levels. My current Pathfinder group is doing the Smuggler's Shiv adventure path and none of the five members chose a Rogue. Having a Rogue's utility would be pretty helpful, I think. It certainly adds a lot of cool stuff, but it ultimately still feels like an extension of the Rogue class. I think it should be an alternate class as the way it's written. I do not think it deserves being a base class as it currently stands.
The Samurai isn't up my alley, personally. I'd imagine it would appeal to the people who would like Cavalier. I don't see enough here in this class to justify it being anything more than a Cavalier archetype. I don't think it should be an alternate class or a base class.
Looking forward to the Magus feats coming out as I'm currently playing one. I'm pretty confident I'll see the +Arcane Pool point feat and/or +Magus Arcana feat after seeing the Gunslinger feat in this preview.
I've seen a few very vocal critics in this thread. I don't get it though. I happen to really enjoy the Magus class and I like the Gunslinger and Ninja as well. I think these three are more interesting than many of the core classes or APG classes.