It costs nothing, on every plan, forever. It is one of the things the Free plan includes and always will.
The check-in is a small card near the top of My Day asking how you are today. It takes under a minute.
Why it matters more than it looks
Two reasons, and the second is the one people underestimate.
It is how you notice yourself. A single day tells you nothing. Three weeks of days can show you that you have slept badly since the clocks changed, which is the kind of thing that is invisible while it is happening and obvious when you look back.
It is how your Circle can see that you checked in, without anyone having to ring and ask, if you choose to share it. Someone who can see you checked in this morning does not need to call to find out whether you are up. They can call because they want to talk to you instead, which is a better phone call for both of you.
You decide whether anyone sees it
Sharing is off until you turn it on, in Settings, under Privacy & Sharing, on the row called My Daily Check-In. Plenty of people keep it entirely private and use it only for themselves. That works too.
Missing days is fine
Nothing bad happens. There is no scolding, no broken-streak sadness, and a missed day is not held against you anywhere.
A check-in you do four days out of seven is worth far more than one you stop because you missed a Tuesday.
It is part of your record
Each check-in counts toward the Your week card on My Day. You can read it back: tap See my check-ins on that card.
There is no target attached to it and nothing to reach. It is simply a record, kept so that a month from now you can see the shape of your weeks rather than trying to remember them.