From: Northwestern Georgia, USA
I can only tell you that the SigInt that the Allies receive is incredibly helpful. I did not think so early in my career as a player of WitP and AE, but I came to realize that eventually. There is a TON of information there if you know how to compile and then interpret it.
Delving more into the nitty gritty, I only rarely receive SigInt about specific capital ships, but it does happen on occasion and can be devastatingly powerful.
The game provides the Allies less SigInt than they got in the war, but combined with other stuff we get that the real Allies didn't get (mouseover intel, "gurgling" noises when enemy ships go under, discolsures from the opponent via email, etc., I think we actually do pretty good.
There is one important way in which we suffer though. Since the game is committed to one day turns, you cannot recall your capital ships from danger like a real commander would. The KB can suddenly show up five hexes away, when in the real war the Allies generally had a very good idea of where the KB was - or at least where it almost certainly wasn't - so that the Allies could often move about with a bit of impunity - especially later in the war. This lack of info, combined with an inability to recall ships in a timely fashion due to the game's 24-hour turn cycle, make it necessary (IMHO) to make liberal use of picket ships. Doing so helps create a much mroe realistic overall situation.