You will look after it, feed it, pay for it (including clawed carpets, sofas, flea repellants, cleaning products to clean where they leave their paw prints and generally making your house unkempt) (and I am talking up and down walls, not just in the kitchen sink when they decide they want a drink; even though they have a bowl of fresh water, but all around it, you will throw food in the bin they get at when you're not looking including even, loaves of bread. 
fleeeas last cost me 70 gbp in products last summer and now I have them again. 
And don't start me on vacuuming their hair even though they are not allowed anywhere other than the kitchen/diner 
Whoever said cats are easy?
Oh and dispensing of the dead animals (birds, mice, frogs and yes, even goldfish) that they bring into the house.
Oh and the fighting with other cats in the garden ffs.
PM me if you need any more info
I have at least another 10 years of it.
Serious post.
On the good side, no more mice and.... that's about the only upside I can think of really.