Canada had the foresight to discontinue their $1 bill when they introduced the $1 coin (Loonie). That pretty much forced the Canucks to used the $1 coin. We had two opportunities to do that here in the US and we failed both times. Americans are very reluctant to give up something they have used their entire life ($1 bill) for something new and unfamiliar ($1 coin).
The fact is it costs the Bureau of Engraving about 4? to produce a $1 bill that has a useful lifespan of 18 months while it costs the Mint 8? to produce a dollar coin that should circulate for at least 30 years. The BOE produces around 4 Billion $1 bills each year at a cost of about $160,000,000. The dollar coin automatically cuts the cost of production by 90% (one coin vs 20 paper notes over 30 years). At a one-time cost of about $320,000,000 the Mint could produce 4 Billion dollar coins that would last 30 years and save the BOE $4.8 Trillion over the 30 years (actually a lot more with an increase in demand over time). That would free up the BOE to concentrate on producing the higher denominations of notes and improve their anti-counterfeiting measures (over 45% of the notes produced by the BOE are $1 bills).
The public would also see a significant cost savings over time. Paper notes must be separated by denomination, properly oriented and counted whereas the dollar coins can be simply weighed.
Personally I prefer to carry a $1 coin that weighs 8.1 grams rather than four 25? coins that together weigh 22.7 grams (2.8 times the weight of the $1 coin).