You can save during a mission,I did it all the time.The game is quite buggy even with the patches,performance issues have mostly been sorted out with the patches but there are still some game glitches that occur.Sometimes you are almost through a mission and then an important "trigger" event fails to happen and you have to redo the whole mission, so it can be bloody annoying at times.It is a pretty good game and the realism is very good,its got a steep learning curve though and there is a sh*tload of controls that you have to get used to and the command interface can be rather complicated.
Cheers