As above, they can attract rats, smell awful and make lots of noise if they aren't managed correctly. To manage them properly takes effort. Ours are in a fully fox proof and rat proof run, and have free range of the garden under supervision. Foxes don't only attack at night - we lost a chicken to a fox (from our urban, 7ft fenced garden) last spring in the afternoon, I'd only popped inside to make a cup of tea.
We clean out the sleeping area and nest boxes every day or so, and the run is dug out and replaced every month. Water and food needs attending to daily at least, more in summer. If the chickens want for anything they are pretty vocal and will let you know about it - ours will shout if they run out of food, for example, so to avoid them disturbing the neighbours with noise you need to keep them happy. They are also vocal when they lay an egg (as would I be!) for a few minutes.
They need a surprising amount of space if kept in a run (I think the recommendation is 2sq m each?) and this can't be skimped on. If they are too cramped they will sometimes fight and attack one another, and also complain, loudly. Ideally the run needs to have the ability to be covered - last winter there was a bird flu outbreak and all chickens had, by law, to be kept undercover for the duration, months on end, as a preventative measure. Many chicken keepers had to build or extend runs and outbuildings to facilitate this, so having an adequate run from the outset would be best. The majority of proprietary arks and runs just aren't big enough.
If you intend to allow them free range they can be fairly destructive. They decimate our vegetable beds if allowed to, and will scratch and destroy a lawn quite quickly. We have a large barked area and chuck seed down, and they like to scratch through this looking for seed and insects which keeps them off the lawn. However I often have to chase them out of planters and stop them eating/sitting on my plants. It's a losing battle. They do surprisingly large poos, in large quantities. You will need to pick these up if you don't want them decorating your garden.
As someone else said, they don't lay constantly. We have four chickens and get an average of three eggs a day, in spring summer and early autumn. Zero in winter. They won't lay their whole life. You can buy hens at layers or at point of lay, or you can buy them as chicks (or hatch your own) but raising them from chicks or eggs takes more equipment and expense. A cockerel is useful to protect the girls, but is obviously noisy - they crow throughout the day and it would be very unfair if you have near neighbours, even if you could keep him from crowing early in the morning.
It's worth looking into rescuing some battery hens if you do intend to keep chickens, rather than buying hens bred for purpose.