- Can it target armor or weapons? Yes. Armor: only shields and Leather or Padded. Weapons: All aside from Pikes and Heavy Crossbows.
- Does it restrain the wearer? Sometimes, except it doesn't cause the restrained condition.
- Is there a save? Yes, for every 10 feet the affected item will be moved. It also takes an action to perform the save.
Immovable Object says that you touch an object that weighs no more than 10 pounds, creatures you designate ... can move the object normally, and you can set a password which suppresses this spell for 1 minute. The object can hold up to 4,000 pounds of weight and a creature can use an action to make a Strength check against your spell save DC to move the object up to 10 feet.
This means that you can restrain guards, causing them to require a Strength saving throw for every 10 feet they intend to move while wearing the item. They can however use a Use An Object special action once on each turn to interact with their helmet. Assuming it has no strap or means of securing it to their head, in most cases they would be able to escape without any real consequence.
As far as Immovable Object goes with armor: only Shields, Leather, and Padded Leather armor can be affected. All other armor listed in PHB is heavier than 10 pounds. Mechanically, the opponent's weapons are the most efficient targets. Depriving your opponent of a powerful weapon attack option can reduce them to unarmed, natural, or improvised weapons.
The spell afflicts its own condition on the target object (and in some rare cases, the user of that object). It does not cause the restrained condition unless your GM makes a houserule.
So this spell's value is ultimately determined by your GM. It's very similar to illusion magic, since your mileage will vary.
For example, one of the GMs in a homebrewing community rules that Bag of Devouring requires some sleight of hand checks when players attempt to use it during combat since there aren't specific combat rules for sticking arms into bags. This spell, however has a range of Touch that permits it to operate during combat (Shocking Grasp would also not work if Touch ranges were disallowed, for example.).
There's some reasonable incredulity at the spell's strength. Yes, it is strong (and can be cast at higher levels). The cantrip Light is a good reference for adjudication, as the spell's text contains the same bug that otherwise exists in weapon buffs such as Holy Weapon which don't explicitly prevent opponent's objects from being targetted. Mileage varies because your GM will make calls appropriate to the game.