We have at backyard chickens for about 10 years now, anywhere from 4 to 10 at any given time. For the most part, we have really enjoyed them. The eggs are amazing (you'll never want store bought again) and they are legitimately fun creatures to have around/watch. The big thing that we did not know going in is that these suckers can be LOUD. We have had chickens that sing their egg song (Youtube that if you don't know what that sounds like) super early in the morning (5:00am) nearly every morning and it gets really annoying to us and can be extremely annoying to neighbors who don't even get eggs out of the deal. Research which chickens are the quietest (though this is no guarantee), make sure you can close the door and keep them cooped up until a reasonable time like 7 or 8, and be mentally prepared to move on from a noisy one... Be considerate of your neighbors if you have any even somewhat close.
Edit: Other thing to consider: you will not be able to introduce new chickens to your flock. They will employ a gruesome pecking order to new chickens. So when you need new chickens someday, you'll need to start completely over.
I'd be happy to answer any questions.
Have you really had that many problems introducing new chickens? We've done it twice, and although the older chickens have picked on the new ones, we've had bullying just as severe or worse within our original flock. I've found that as long as you have >2 new chickens, and cordon them off for a little while where they can see and kind of interact with the original flock before you really put them together, it works out alright. You definitely can't brood or introduce single birds, for sure.
We've had chickens for 5 years now, both in a small backyard in town and in our acre lot now. If you already have something to use for a coop, that's awesome!
You'll need to think about brooding chicks for about 12 weeks (until it's warm enough/they're big enough to go outside). They need to be kept really warm for the first few weeks, no drafts, with a heat lamp, but after they're mature they're SUPER easy. Under no circumstances keep them in your house the whole time you're brooding them, because chicken bedding is stupid dusty, and you don't want aerosolized chicken poop just wafting around and coating everything. Trust me, it's gross forever.
My only real complaints are that their fresh poops are so so so smelly, and that you have to lock them in at night and let them out in the morning. Otherwise, their food is very inexpensive, they eat all of our food scraps that we don't compost, and they're pretty funny to watch :)
Make sure they're legal to keep within your city limits, and whether or not you need to license yourself. You may also want to check with your neighbors to be sure they won't mind, especially if you have an HOA.
We chose our chickens based on 1) Weather tolerance 2) Egg color 3) Egg laying capacity and 4) Appearance. We have a little bit of overlap, but our chickens are mostly very different from one another, and it's super fun :)
We have three Ameraucanas of varying color (blue eggs), an olive egger (green/olive eggs), a Brabanter (white eggs, but they have a mohawk/head poof of feathers that's amazing), a Marans and a Welsumer (dark brown eggs), and a variety of "regular" brown egg layers - Orpington, Brahma, Speckled Sussex, Barred Rock, Gold Sex link, etc. Our 4 year old loves that he can tell which chicken is which to name them, and I love having such a variety of eggs.