Challenging, seldom frustrating.
It wasn't really the quick looks or the lets plays that made me interested in FTL because when you watch it with untrained eyes it is really quite stale, impressive with all of its micromanagement and the administrative look of it sure, but it's just not very visual.
That said, I am so glad that the good things I heard about it convinced me to buy it(It helped that it was cheap and would work fine on my potpal) because this baby is the Sci-Fi'est Sci-Fi that ever Sci-Fi'ed. And I love my Sci-Fi. The delegation of crew, the management of the limited powersupply and the random elements that makes the short but highly repeatable journey feel alive make for an adrenaline filled ride.
And it's tough, very tough. I highly doubt that I'll ever play on normal difficulty again.