It is useful, especially with single coils and even more so with P90s with their large wide coils.
It won’t stop all hum, but it can cut down the level quite considerably.
But to be effective, the copper needs to be grounded and it needs to enclose as much of the electrics as possible. You can’t do too much about enclosing more of the pickup cavity than just the bottom and sides, but in the control cavity, you can also stick it on the underside of the cavity cover. On guitars/basses with pickguards that mount or surround the pickups, you can cover the underside of the pickguard as well.
The copper will be automatically grounded where it touches the pots, but for separate pickup cavities, you’ll need to run grounding wires out from already grounded shielding areas. I use thin wire and stick the bare ends down with more copper tape. For cavity covers and pickguards, you can ground by copper to copper contact, which means running the foil up and over the cavity recesses, and for pickguards, over the top of the body, but underneath the pickguard so it can’t be seen. Ideally run it on pickguards to screw areas so the copper surfaces are firmly held together at those points. Large areas of plastic tend to warp slightly, so trusting to a couple of small areas in the middle of a pickguard to make contact isn’t the most reliable of methods.
Because the shielding is grounded (it really doesn’t do much for noise if it isn’t) then beware of signal wires touching it with the result of no audio output. A single loose whisker of wire can result in silence. So be careful with your soldering and minimise the amount of exposed signal wire. Use heat shrink where possible. Use insulating tape over the foil where contact may be made once you put a cover or scratchplate on. Especially take care around the jack socket area as the spring contacts will move outwards when a plug is inserted, so it’s often safest to avoid shielding the hole around side-mounted jacks (and definitely use insulating tape in Strat jack socket cavities).
If you need more copper foil, you can find cheap rolls of shielding tape around 25-30mm wide on eBay or Amazon if you can’t find it locally. Also sold as slug-repellant tape (same product, different name). Just make sure it has conductive adhesive. The majority of tapes do now, but don’t assume and avoid anything that doesn’t state it explicitly, otherwise you may need to solder the strips together to get continuity.
And best to use a multimeter to check for continuity between strips. Even with conductive adhesive, I’ve sometimes had to run a crosswise strip of tape over the top of overlapping parallel strips of tape in order to get continuity from one end of a pickguard to the other.