The US, like Canada tried to introduce a $1 coin awhile back to reduce minting costs of the $1 paper bill. This was successful in Canada but not so much in the States. Why was it not widely adopted? Are these coins still available in circulation?
I like the coins. (They're fun to leave as tips.)
I think almost everyone else I know thinks they're a pain in the butt. They are a bit more inconvenient than dollar bills.
As for why they're often in post offices, they have to be. The Presidential $1 Coin Act of 2005 mandates that vending machines at various types of businesses accept them.
They are certainly still in circulation, and there are new versions being minted. I think one of the big reasons they haven't caught on, is that not many vending machines have been retrofitted to take the new coin, while they do take the one dollar bill. Also many people here are just not comfortable with the new coin.
The current coins are the presidential series. 40+ coins issued 4 per year. You can get them in rolls of 25 at the bank or order on line. I use them, but have never gotten one back in change. On a side note, years ago I ordered a pack of $2 bills from my bank, I think they came 1000 at a time. The lunch lady at school asked my daughter to stop using them, she had no space. The toll takers hated them, same reason, I guess. Given the cost of the coin vs paper and how long each last, they should stop printing $1 bills here.
Why was it not widely adopted?
I think there are a couple of key differences.
The first is that the "susan B" dollar coin was regarded as unattractive and easy to confuse with a quarter. When canada released the loonie a few years later they learned from this mistake and made the coin in a style that was more fitting for a high denomination coin while still being similar enough to the US one to make life easy for vending machine makers.
Secondly as far as I can tell Canada forced the issue. A couple of years after introducing the loonie they stopped issuing dollar bills. So those requiring a $1 denomination for making change had no choice but to accept the coins. The US on the other hand kept issuing dollar bills and merchants kept ordering them in preference to coins. The US was left with a big stockpile of dollar coins and stopped minting new ones while it worked thorugh the stockpile.
Are these coins still available in circulation?
While the "Susan B" was not a wild success it did see some use from the vending machine and mass transit industries and the stockpile was gradually depleted. This resulted in a final run of Susan B dollars in 1999 followed by the introduction of the "Sacawega" dollar in 2000.
The US had another go at trying to push dollar coins in the late 2000s with various native american and presidential designs in the hope of stirring up interest and getting people to use the coins but again they didn't force the issue, again reception was lackluster and again they ended up with a big stockpile of coins.
As I understand it the US is currently still issuing dollar coins from the stockpile built up in the late 2000s. They are also minting new designs in reduced numbers for sale to collectors.
Not many folks like them, so that's why you see so few in circulation, but what is astounding is that the US government made so many of them. Old article here reporting $1 billion of those coins sitting unused in Federal Reserve vaults in 2011, expected to grow to $2 billion by 2016 because of laws mandating their production.