Bonds

Are bonds ever useful? I'm tempted to use one but I don't know whether it would be actually useful in the long term.

Yes, use it.

Similarly to what Jamesthomeson said. Once your airline has grown and has a good DOP they are completely useless; if anything they hinder your progress because they limit how many aircraft how can lease (unless your airline is making enough money and don't go negative). 

At the beginning of the airline, they're useful to an extent. Get to many bonds while often defaulting on the interest and sure enough they'll hinder your ability to lease more. I usually get 1 5m and 1 2.5m one, which reduces leases to 100 (which is enough for a new airline), and as time goes this number goes up. 

Getting them depends on the kind of airline that you run and the type of challenge that you want at the beginning of the game. 

LATAM

Is there a way to pay off a bond early?

Nope, just keep making the interest payments and eventually it will be paid off

(i know few) bonds for real airlines are very difficult. some times banks offers ridiculous sums to airlines. Because aircraft cost many $$$ and Airlines arent normal enterprises.