(Stolen from Unearthly Item Buying Guide, which was very useful here.)
Crystalline flame
Most of the content in this item requires you to be Etheric, so it will probably not see much use in-run unless you're deliberately going out of your way in pursuit of a conduct or something. You can get the Crystal Strike technique without being Etheric. At chain 2, it could be handy if you're running a Melee deck and have not yet gotten any instances of Brutality. This technique is also handy for spading out opponent Will values, if you're into that sort of thing.
The other techniques this item grants are pretty potent if you are planning to use Etheric techniques. If you happen to have Fae Eyes, Defensive Burn is particularly nice. Fist of the Herald, available to Fae- and fish-aligned alike, is powerful, if a bit Body-Intensive. It's also worth pointing out that the drowning flame is still currently best-in-slot for Etheric Power. Both the drowning flame and the serene crystal give +1 Eclipse Duration. This bonus has been eclipsed by the charmed chain in the seasons since this item was first released, but if you don't have a chain those are currently the only other sources of that bonus from the weapon equipment slot.
Kinak says there is still something interesting we could learn by carefully looking at the messages you get when you [use] a drowning flame or serene crystal. Other than that, this is one of the very few unearthly items in the game that are fully understood.
Spacious cargo pants
Good pants are hard to get in-run, so if you're resetting these pants are attractive just for the +2 Melee Defense. Getting the other benefits from the pants requires you to Check Your Pockets. This involves some trade-offs. For one thing, at chain 4, this technique conflicts with the all-important Dive for Cover, so there's an element of risk and uncertainty involved in using it. Furthermore, you have to be Etheric to get the really powerful rewards from the pants. Still, the gamble is probably worth taking if you are primarily using Ranged combat. The Lucky Shot technique is available even if you are not Etheric. While it's not guaranteed to do more than its low base damage unless you get, well, lucky, it does let you make long chains very early on. The rest of the non-Etheric pocket items aren't especially impressive, although the serviceable bandages are always nice to have.
The Etheric-only items and techniques are another story, even if you have to wait until post-run to get them. The "Bullet for … " techniques do solid damage and, more importantly, allow you to do damage and potentially block attacks with the same technique. They're also nice to have given the dearth of chain 6 Ranged techniques that work well as filler. Bullet for Surt is especially nice, since there are many opponents who use all or mostly Melee techniques. In gang warfare you are more likely to encounter opponents with Stealth- and Etheric-heavy decks that would allow you to use Bullet for Ginn and Bullet for Ymir to good effect. Unfortunately, the "Bullet for … " techniques do not seem to work while underwater.
The knuckle bones are still a bit of a mystery. There is likely something more to the knuckle strands, but we have yet to figure out what.
Book of Common Dreams
The Common Dreams skill this item grants is almost pure powercreep. Once learned, it will be available in every season, even when the book has rotated out of Zack's House of Coins. Using the skill costs only one energy of Etheric and no turns. Unlike the summoning skills, its cost does not increase as you use it more times in a day (and it does not affect the cost of summoning skills no matter how many times you use it). Another difference from the summoning skills is that you cannot control exactly when it fires, so it's a much less precise way of burning off energy of Etheric. Still, while the bonuses this item grants may not be overwhelmingly powerful, in many situations the opportunity cost to getting them is negligible.
In-run, this item is potentially helpful, but with no apparent way to control which dreams/items you get, you can't really plan a strategy around getting any particular item. If you are extremely lucky, you might be able to score a carnivorous beetle, which could help considerably with the Fifth Floor. More commonly, you'll wind up with at least one stat-boosting item, which give hefty boosts for the various points in your run where attributes matter; these can be particularly helpful for passing the Low Road. Again, though, you may not get the item you're seeking. Since in-run you don't generally get Etheric without a fairly specific goal in mind, you may not have a great deal of energy of Etheric to spare for getting dream items. On the other hand, if you are accustomed to using summoning skills several times a day, grabbing a dream item or two might be more a case of "can't hurt, might help".
Post-run, the stat bonuses are not likely to be as important. This is likely true even if the bonuses are doubled, especially since you may well not want to devote your offhand slot to a dream item in order to get the double bonus. If you don't have a Midgard blastshield or one of the limited-time items that provides +4 Fire Defense, the charred wooden souvenir can be a valuable piece of equipment for post-run use. Otherwise, there are also generally more potent alternatives to the dream offhand items as far as defense goes, although the Chance of Etheric Encounters modifier they grant may be a compelling bonus in some cases. One final point for post-run use is that (again, as far as we know) once you have one of each offhand item there is no benefit to having additional ones.
Lens focused on the unknown
If you're using ranged techs, this is a very useful item in-run and doesn't require a very large deck to work with. The lens focused on the distance in particular provides buffs to both ranged power and power to techniques using perception. Other forms can also provide stealth defense or etheric defense, which can be tricky to come by in run. Even post-run, the lens focused on minutiae is at the time of writing the only source of stealth defense in the gadget slot.
The 'exaltations' are still not well understood, but they can provide XP or energy of Fluid Focus, which can allow you to make use of etheric power bonuses that might otherwise not be helpful in a ranged or fire deck making use of perception.
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I like the lens of the unknown a lot for use in-run. As usual, the skill-books (Common Dreams) are going to be less useful if you already have them.