Well-Maintained is a card multiplier, it's power is strictly dependent on the card you apply it to so evaluating Well-Maintained in vacuum is HARD. Even so, here are some notes to keep in mind:

• It returns the cards to your hand when discarded, this means that cards that do not self-discard are not returned till manually or unintentionally discarded. The card is missing a clause that let's it discard the attached asset, in my mind.
• There are several treacheries that discard assets, such as Crypt Chill, that Well-Maintained helps you defend against.
• Costs are not refunded so you still must be able to pay for the asset again, an issue mostly for the big guns.
• Custom Ammunition and Reliable are the only/most important Upgrades to be released so far.

The most immediate way to read Well-Maintained is to assume it's meant for a big gun, but there are lots of cards that operate far more smoothly within the bounds of the wording. For example:

• Mk 1 Grenades self discard, thus you regain them without any extra actions or costs.
• Disc of Itzamna and Police Badge discard as part of their mechanics, the immediate re-occurrence is fantastic. This is also true for all items that grant health or horror, for example Bulletproof Vest, Elder Sign Amulet, Key of Ys.
• Some more notable self-discarding items: Backpack, Baseball Bat, Colt Vest Pocket, Gravedigger's Shovel, Grotesque Statue, Kerosene, Knife, Lantern, Lockpicks.

There are lots of items that don't discard but still work good with Well-Maintained, it just requires an extra step to execute on, to recurse an empty gun you need to play something else over it in the slot, for example a Flashlight or another copy of the gun, this is still a rather slow process so I wouldn't pick Well-Maintained for this purpose exclusively, Extra Ammunition is a much better option for reloading guns! But, when you have some of the other cards mentioned above, and/or one or both of the upgrade cards, Well-Maintained becomes a much more impressive addition.

Finally there's a couple more combos that are absolutely worth your attention:

• Any large gun + Well-Maintained + Act of Desperation. Recurse the cost and the gun you just sacrificed, ready to be played once more.
• Lupara + Well-Maintained, this combo is a bit slower but it's pretty robust, a good fit in a deck that's investigating with a full set of Lockpicks and Flashlights.
• .45 Thompson + Well-Maintained. The gun pays for itself, ready to do it again once you're able to discard it. A very natural combo.

I think Jury-Rig raises the usefulness of this card even further for the 6 investigators who can take both without buying Versatile (and I wouldn't buy Versatile for a single copy of Jury-Rig):

• Carson Sinclair
• Tommy Muldoon
• Zoey Samaras
• Charlie Kane
• Lola Hayes
• William Yorick

While I typically don't like assembling a three-card combo, Old Keyring + Jury-Rig + Well-Maintained is pretty strong. You won't get Well-Maintained back, but you will get the Keyring and Jury-Rig back.

Lantern only gives you -1 shroud instead of Keyring's -2, I think it's better for the Investigate combo because of no Uses (X) and you can discard the Lantern on demand, which allows you to wait until the combo is assembled and then "recharge" your Jury-Rig when empty.

Any Firearm + Jury-Rig + Well-Maintained is a great combo with this and Jury-Rig, as it allows you to recharge the Firearm AND the Jury-Rig, and obviously synergizes really well with Act of Desperation.

A Well-Maintained Backpack is really interesting for investigators who need to find their items and supplies, and / or those wanting to search their decks (anyone who can take this and Astounding Revelation or Surprising Find)

While this card can be used by guardians as insurance against item destruction on their expensive weapons (lightning gun, timeworn brand), paying the cost of 4 or 5 resources to replay the item could still be problematic ('act of desperation' could mitigate this by giving you resources equal to the item's cost after a successful attack).

A much better use, in my opinion, is to play 'well-maintained' on low-cost and high-value items that are cheap to replay and preferably discard themselves after use. One such item is 'grotesque statue' in the mystic class. Diana Stanley can take both 'well-maintained' and 'grotesque statue'. Once you have a statue in play, the two 'well-maintained' events basically function as extra copies of the statue, so you're functionally playing a Diana deck with 4 copies of 'grotesque statue'! Lola Hayes can take both cards as well, but she needs to be a mystic to trigger the statue and a guardian to trigger 'well-maintained' which might be difficult to achieve consistently.

arkhamdb.com

Overpowered in a Vincent Lee recursion deck with the combo piece Bandages + Well-Maintained + Spirit of Humanity. Endless recursion of Bandages, fill the bag up with 10 Blessed Tokens with Spirit of Humanity. This also procs Vincent's ability, giving you endless On the Mend (Unexpected Courage). You'll need someway to offset the horror damage, though.