This game simply delivers. A lot.
It's a hard game? I don't think so. I've just finished it on hard difficulty and i hadn't more problems than i had with other similar games. You have to play this game like the old school fps games: learn how to use the full space of the arenas in which you have to fight and you'll only die when you deserve it. Which isn't a lot of times, if you are a decent player. No sign of frustration through the entire run for me.
If you have so much trouble, you simply don't spend time to find all the secrets and the money: you can almost buy everything in a single run if you search carefully, and it's also a ton of fun to do that.
It's a short game? Not more than, say, Modern Warfare. If you want to rush through the game, running and gunning at everything while you escape... Sorry, it's not conceived to do that. You have to clear areas using both your reflexes and your brain, playing with everything that the game offers to you. That's what the programmers wanted you to do.
I can agree about the very thin story, the bad performance of the voice actors and the end, which is completely anti-climatic (the big giant that you have to fight at the end of act V would have been a much better final boss), but when it comes to gameplay, well.... it simply does everything right for a 2011 old school first person shooter. Every weapon is good, every gunfight is pure adrenaline, interaction with the environment is spectacular....
If you manage to not have fun with this game, you seriously have to consider to change your hobby, because this is a videogame in the purest sense of the word.