I think the title says everything, how secure is it to use apt-get in a network with an attacker who injects his own packets and blocks the original packets (man in the middle).
All of packages within the Ubuntu and Debian apt repositories are signed using SecureApt. Asymmetric cryptography is very secure and its highly unlikely that you will be compromised in this way. 3rd party repositories are hit and miss, they may or may not be singed. If they aren't signed, apt should throw a warning informing you that you are installing an unsigned package. But if its from a 3rd party you could also just be installing a backdoor, so make sure you trust that source.
On a side note, people don't really "inject packets" to perform MITM. They are going to use something like DNS cache poisoning or ARP table poisoning so that you download the package from an HTTP server that they control. This isn't an injection attack.