It's a significant improvement over backstab.
I don't think so. I'm not a fan of bonus damage dice, for a variety of reasons I know I've expounded upon recently. It devalues base damage, devalues critical hits, devalues the tactical setup, and creates some wonky damage math. This, I've fixed, by creating a variety of abilities that hybridize SA, backstab, and some other concepts.
I'm also not a fan of it being mandatory. From the start, the thief concept and backstab were only partially concordant. But the transition to rogue broadened the class's niche. Many rogues are not sneaky combatants. Indeed, many are neither of those things. And yet, even with its numerous archetypes, PF really doesn't touch SA at all; every rogue apparently has it. At the very least, there should be a sub for a few other ways of approaching combat, like Skirmish, but perhaps for other avenues of advancement as well.
Admittedly, I haven't fixed this issue. It's a big one.
No more so than any other family. For example, in my game a party consisting of clerics of Jord, Sesstra, Aynwen, Arattay, Lado, and Aymira would probably get along like peas in a pod.
It's certainly possible to have clerics of compatible deities get along. However, I would think that the specificity of their beliefs would cause more conflicts than a conventional party, and even more so if you start getting into deities of radically different alignments.
I don't feel that way. Aside from overcompensating for the general mediocrity of the 1e/2e cleric, the cleric is IMO one of the best designed classes in 3e. It can be subtly diversified by a very elegant mechanism. I see the need for very few changes.
The domain concept is good, but was never adequately implemented. Domain spells shouldn't be just one bonus slot a day, they should be a large part of the character's spellcasting. Domain abilities should be more substantive (even more than PF). And instead of the standard BAB/saves/skills/proficiencies/hp package, these parameters should vary by deity. A cleric of Kord maybe should be getting armor and weapon profs and good fort saves, but not good will. A cleric of Boccob really should be more like the cloistered cleric variant.
Moreover, there should be a clerical sorcerer. Preparation really is obsolete for divine casters. The favored soul was kind of dumb, and the oracle is nice but not quite on the mark for that purpose. Even 5e promised us a priest class and backed out.
I guess you should avoid playing with a jerk for a DM. Either that, or you should pick the sort of deity that you'd really want to serve so that you don't end up with a personality conflict between you and your boss - I often suggest a god of hearty good humor and drunken carousing to anyone I think isn't of a philosophical inclination. I could get really cynical here, but I'll refrain.
I don't think it takes much jerkiness from the DM to make it a very serious consideration. If you're not a cleric, you're your own boss. If you are, you're giving that up. To me, that's the real reason clerics weren't popular, not because they were too mechanically weak.