Let's start out with the feat description, taken verbatim from the source (Players Handbook II -- it really should be Player's Handbook II... that's driving me nuts) for the purpose of debating it.
When you strike a an opponent's vital areas, you draw on your ability to land crippling blows to make the most of your attack.
Prerequisities: Skirmish or sneak attack ability.
Benefit: When you score a critical hit against a target, you deal your skirmish or sneak attack damage in addition to the damage from a critical hit. Your critical multiplier applies only to your normal damage, not your skirmish or sneak attack damage. This benefit affects both melee and ranged attacks.
Now, the powergamer in me likes this feat -- it's a good way to allow a rogue the chance to do their 'normal' damage on a target they're unable to feint or flank.
The, er, problem (and I used that term very loosely), is that, in the RAW (rules as written), they stack -- you can hit a flat-footed opponent and score a telling blow while doing so. This can seriously ramp up damage, especially with someone with 'hot dice' and a keen 18-20 weapon (or, say, improved critical in a similar weapon), especially if they have some way to greatly improve their odds of hitting.
Case in point -- in our Monday night D&D game, we've hit the level 20 range recently (like I said -- high level) and I play a Rogue/Fighter/Assassin/Tel'flammar Shadowlord who runs around with a Keen, Deadly Precision, Kukri of Subtlety (I was going to go with the sword, but the kukri is just so much more stylish) in her offhand, and a sentient rapier in the offhand (also with Deadly Precision, but sans keen and other such cheaty mechanisms).
With this build, I've only got a base of 5d6 sneak attack (Tel'Flammar does not get any sneak dice, to my chagrin), but Deadly Precision bumps this up to 7d6 and a Rogue's Vest bumps it up to 8d6 (putting me on par with a straight 15th level rogue).
I may be overestimating it, but in the surprise/first round of combat, I can (and, so far, have) cranked out a pretty crazy amount of damage since snagging this feat (at my fifteenth hit die). Granted, my d20 is pretty hot whenever sneak dice are included in the result (and not so hot at everything else -- saving throws included), so I tend to get a lot of these "double sneaks". It just seems so overpowered, especially if you can find a way to guarantee that your critical threats will land and become critical hits against most foes (a wraithstrike spell, brilliant energy weapon, or using something like a flesh ring of scorn).
Doing some reading over at the Wizards boards, the people seem to be split into two camps -- the RAW folks, who think that it's a powerful feat but should work as written, and those who believe the 'real intent' was simply to increase the set of conditions under which you can make a sneak attack to include 'successful critical hit'. The latter also use the justification that since the feat doesn't explicitly say that they do stack, they don't stack (which seems backwards -- WotC books are usually pretty clear when things should not stack).
My GM, to provide a counter point, can visualize the effect of a 'double sneak' from a flavour point of view -- not only do you hit the organ your sneak attack was aiming for, but you hit in an extremely brutal manner (either twisting and making the wound worse, or, to use his example, puncturing one lung and blowing through and puncturing the other).
Since there hasn't been an official word on this (nor any errata that I've seen), I just wanted to throw it out there.
a) Do you think this feat, as written, is too good?
b) If so, what would do to "fix" it?
**** EDIT TO ADD ****
Sneak Attack Of Opportunity [Epic] [link]
Prerequisites: Sneak attack +8d6, opportunist class feature.
Benefit: Any attack of opportunity you make is considered a sneak attack.
**** END EDIT ****
To be honest, I think the sheer number of attacks I'm getting off in a round (two weapon fighting, shadow stride + shadow pounce, shadow jump 5' + shadow pounce) is the problem moreso than this feat, but the feat in and of itself seems pretty harsh.
What are your thoughts?