This is a very useful and potentially a very versatile card, though in a majority of cases its versatility is disregarded and it just gets joined at the hip to Core Gandalf - it can work well in many more contexts than that though. The effect is simple - at a cost of 1 resource you put an ally into play for a single phase. Since most allies cost more than 1, you're most likely getting an effective discount, perhaps using an ally you couldn't afford to play normally, but you only get to use them for that one phase (so most likely they only get one action) before returning them to your hand. It should be noted that there is no limitation on the sphere of the ally, so this is a means by which you can bring in out of sphere allies, albeit temporarily.
The utility of bringing an ally into play for just one phase is obviously twofold - you get their stats and you get whatever ability they have, so any ally which has something particularly good in either category may be a good candidate for Sneak Attack. In many cases of course allies with high stats also have pretty useful abilities, though what makes an ability useful for a Sneak Attack is not necessarily the same as what makes it useful for playing the ally normally. As far as stats go, options do not spring to mind for , since it's fairly rare for an ally to have more than 2 , though it's still an option if you're particularly short on willpower. I would generally prefer something which would allow me to adjust my after staging though. For and there are better choices, and it's easier to justify it as you have more available information - you know if you have one more enemy than your available good defences, or if you just need a little more to kill an enemy, whereas Sneak Attacking an ally for questing is always more speculative unless you've scried the encounter deck. So combat is friendlier to Sneak Attack than questing if you're just looking for extra stats, and there are plenty of options there.
The more interesting case however tends to be considering the abilities of the allies. Now a lot of allies won't get much use of their ability in a Sneak Attack because the benefit of a lot of ally abilities is that they're repeatable, which doesn't apply if they're only in play for the one phase. There are three types of ability which tend to be particularly friendly to Sneak Attack - action advantage, enters play abilities, and discard abilities (or other means of leaving play):
- Action advantage is obviously a combat thing - allies like Boromir if you can keep him healed up, or Vigilant Dúnadan if there's a side-quest in the victory display (and you can keep him healed up) can defend a long line of enemies, thus getting you much more value out of the Sneak Attack than if you brought in a regular ally who'd just exhaust to defend or attack once and that's it.
- Enters play abilities obviously work great with Sneak Attack, as you can trigger them without spending the full cost of the ally this way. This is why Core Gandalf is such a good target, because he gives a choice of three great enters play abilities on top of his incredible statline. Elrond is another great choice for basically the same reason. Sarn Ford Sentry can be a good one in a Dunedain deck to draw a load of cards. Descendant of Thorondor can do some nice direct damage, and using Sneak Attack for it avoids the problem with playing it that the enemy you want to damage may not still be in the staging area come the Planning phase; likewise in an Eagle deck Gwaihir triggers on entering play. Sneak Attack can also be a fantastic inclusion in a Silvan deck because the archetype runs on allies with enters play abilities, on top of which they also get the stat boosts from Celeborn when they enter play.
- Discard(/other leave play) abilities likewise - you can trigger them without having spent the full cost of the ally. Beorn is a fantastic one since you can trigger his +5 and then choose to resolve Sneak Attack returning him to hand before his own effect shuffling him into your deck. For lower impact but not insignificant options, there are things like the Eryn Galen Settler, Grimbold becomes a Feint for the same cost, Honour Guard if you're in Valour, Sneak Attack Westfold Horse-Breaker readies a hero in an emergency, or Westfold Outrider can engage an enemy to prevent an attack or remove threat from the staging area as needed. Continuing on Rohan, Escort from Edoras and Háma both automatically discard themselves after doing something useful, so if you're going to do that there's certainly value in getting them into play cheaper, while Éomund triggers on leaving play so if you Sneak Attack him for questing he will ready all Rohan characters in play when he returns to your hand at the end of the phase. Eagles are also great for Sneak Attack because of the leaves play thing - Descendant of Thorondor triggers again on leaving play, Vassal of the Windlord and Winged Guardian both automatically leave play after one combat action so you're not losing out on anything by sneaking them in, and any Eagle ally you Sneak in can go under Eagles of the Misty Mountains instead of going back in your hand; for one which also works in a non-Eagle context, Landroval can save a hero from death by returning to hand, so bringing him in with Sneak Attack is a great option to have. Finally, I have to mention Silvans again, because in addition to the enters play effects the other key aspect of a Silvan deck is using the events to return them to hand, and you might as well trigger that with Sneak Attacked ally who's going to return to hand at the end of the phase anyway - the ultimate case of this would be using Sneak Attack to bring in a Silvan ally, triggering their enters play effect, having them do something, and then before the end of the phase returning them to hand with The Tree People to replace them with a permanent ally.
I should also mention Faramir, who doesn't fit any of the above categories but can still be well worth a Sneak Attack because his ability is so potent for the amount of it can add to your questing, and that after staging.
As I said, Sneak Attack is often only considered only in the context of Gandalf, and admittedly Sneak Attack Gandalf is one of the oldest and still one of the most powerful combos in the game. In some ways I wish the two didn't synergise as well as they do, because then Sneak Attack might be appreciated more for the other options it offers, but even with Gandalf in the mix, you should always bear in mind the many, many other uses of Sneak Attack, including but not limited to the ones I've brought up here - and Sneak Attack is also the sort of card which once you've put it in your deck for a specific reason may then end up being used for a different reason in an emergency, and it can really be a game-saver under those conditions. Flexibility really not to be dismissed.