How insecure… really… is FTP
It's as insecure as your network is. If the network can be sniffed (WiFi, rogue ISP, etc), finding the password and data is as trivial as running wireshark. Given a point to listen on, you could train a bright 10 year old to get the password and data in under an hour.
What about a user's credentials, password, etc.?
All completely in the clear, with very easy tools available to snarf them.
Granted the particular network that one is on is also of importance. A secure office network is a bit different from an airport lounge.
True. As I said, FTP is as secure as the network is, and no more. It's also a matter of the threat model. Who's going to try to get your FTP data, and why?
These days with sftp and other ways of transferring data securely, it's more of a question of why you would use ftp than why you'd use the secure alternatives.
I think what you should keep in mind is that these types of scenarios only tend to balloon with time. You start out not particularly caring about the data and having your credentials be snarfed, but what you're doing can morph over time into something you really wish you had spent the time to just do it right in the first place. Many a IT professional has been told "it's only ever going to do X, and it'll never be used for Y" only to have that same person come back 18 months later and say "I know I said it'd never do Y... buy we really need it now. Can you fix it?"
If you have any choice in the matter, it's almost never a good idea to start out insecure, and just assume "we'll add security later when we need it.". The secure alternatives aren't really any more difficult to use and maintain, so why wouldn't you use them?