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Pro Preserve your session
Screen allows multiple terminals (screens) within a single screen number, so when you reconnect they will all be there.
Pro Supports shared sessions
More than one user can be connected to a single screen session.
Pro Also works as serial port terminal
This is actually very valuable for embedded or networking work.
Con Scrolling becomes tricky
You can no longer scroll with your mouse wheel, unless you do Ctra-a + Escape first
Con Doesn't work too well with ssh X connections
If you have an X connection over ssh within screen, it likely won't reconnect when you reconnect your screen.