The man upstairs said he would share the cable with me. What are the advantages and disadvantages?

Of course he just wants to save money, and then you save a lot.

But if you surf the Internet at the same time, the speed will be slower. If one of you doesn't surf the Internet, it will be the same as before. It doesn't matter if you slow down. ADSL should be able to handle two, but if you like to download things everywhere, you'd better not agree, which will affect the download speed.