Alright alright I'm sick today and didn't want to expend the energy, but here you go.
It's not perfect, mostly because of metadata concerns (the encryption itself is top notch), and there are alternatives that are even better in terms of anonymity, but no normal person would likely use them.
You could sum it up as this: The contents of your conversations will be secure while transmitted over the internet, but the server knows how often and to whom you speak to, and also where you are (physically) as long as you have network connectivity.
Put another way, your contact list and contact frequency is revealed to the server, altough the contents of the communications remain secure (private).
You can improve on the physical tracking part of the situation by proxying Signal through Orbot (Tor client) on Android - not sure if calls work like this, messages surely do.
LibreSignal, a FOSS fork that enabled non-GCM users to talk to normal Signal users, also contained several patches that at least enabled you to filter which contacts are sent to the server, not sure if that's there with the latest Signal version.
tl;dr: for most people this is the best private messenging app available atm
