I think that saying its for fans of the series is totally fair. Personally, I'm loving it, having finished once, and on my second playthrough so I can end up buying all the traps. That said, I've been a fan since day one and I always complain that they should put back the ability to make monsters out of your kills, and yes, unfortunately the trap customization isn't nearly as good as Deception 3.
The AI isn't great, no way, but I think Deception has always been one of those 'how can i outdo myself this time?' games rather than a player vs. AI situation. Well worth the cash for me, if you're not sure if this is for you, I'd advise you to wait until it hits $30.
I'm still puzzled as to why it took five full years to get a new title in the series when we saw three on PS1 alone.