The Athena campaign is actually as long as most single player games are these days. What drags it down IMO, is that the "re-mastered" Butcher Bay campaign isn't all that re-mastered. The lighting and such are definitely improved but I still feel like I'm playing a four year old game. It actually sort of feels like they re-used some assets from BB to make Dark Athena and I was rather hoping for the opposite. To top this off, the performance, while still perfectly fine on a modest computer, doesn't seem as good as it should be considering the very average graphics. I'm getting framerates in the mid 40's but from how it looks it seems I ought to easily be getting 60+ fps.
That said, I rarely feel as though I get my $50 worth from most single player games.
All told though, it's a solid game. Anyone who is on the fence is probably better off waiting until the price comes down. I'm happy with it though. It's still as fun as the original and that's not half bad.