I have to respectfully disagree on most, if not all, points. The DLC was never intended to advance the main story. Sure it's a cotinuation of sorts, but that's all it was meant to be. After all, if there is a sequel, why should people have to have played the DLC in order to understand how it starts? The whole point of the 'Specials' is that they will occur between Season 1 and Season 2 but won't be necessary in order to understand the plot of either. I don't really understand why anyone would have been expecting some kind of story revelations from that.
Gameplay-wise, I agree that it was harder and more hectic than the earlier episodes, but that's kinda the point. It is completely in keeping with the setting and the story that such a scenario would play out in such a way. Man it's hard to write this without spoilers. Basically, it's hard but it's not THAT hard, and the pacing is not all that different from some of the longer sections where the original game throws lots of enemies at you in a row. In the signal, at least that same kind of pacing is framed with some very satisfying surreal imagery and level progression. TVs scattered throughout The Signal do the same kind of job as the manuscript pages did in the original game, only this time that storytelling device is used in a much more visceral and immediately threatening way. We literally get to witness a violently insane force doing everything it can to tear Alan apart both physically and mentally. Sure it's a change of pace, and there is hardly any breathing room, but it's a welcome change. Anyone who wanted the original game to be a little more freaky and genuinely scary will be pretty happy with the DLC so far.
And if you think about a few plot elements (not neccessarily just what you see in the cutscenes), there are a good few subtle clues regarding the main story.
Personally, although The Signal is a little story-lite in terms of the main story and the cutscenes, I would still give it a solid 8 or 9 out of ten. In my opinion (and that of a good few reviewers, as you'll notice if you google it), The Signal is an excellent add-on and an example of how all DLC should be done.
No offense to Susan, but I implore anyone who is sitting on the fence about the DLC to ignore this review and make their own mind up.