Adverts…I wouldn’t pay extra to lose them…it’s a static screen when the Kindle is asleep, normally just another book they think you’ll like. Occasionally it’ll offer you a £50 kindle cover or charger etc.
I think I’m on about 150 books so far and I’ve previously clicked through the ad about three times.
One of the best things about the kindle is you can read the first chapter or three as a sample, if the book doesn’t grab you, you can just move on and try something else rather than wasting £3 or £4 on it. Strangely my wife recently admitted she *never* uses this feature! I find it quite useful, I like travel books and sometimes they can be pretty boring!
We also use the same Amazon account for both devices which works fine, the only time it gets confused is if you both overlap reading the same book, one of you has to go on airplane mode otherwise it tries to sync to different pages