I don't think the game is necessarily challenging for all parties, it really depends on who you choose to bring along. What's nice though is that it will throw completely different types of enemies at you so you have to stay on your feet. On one of my playthroughs I had three mages in my party at one point and got absolutely hammered by Possessed Templars in the Tower and by Reavers once they gained staff immunity, as well as later on in situations involving scattershot (grrr). For a challenge, I'm going to reinstall the game over Easter Break and make a no-mage squad with 3 dual wielding rogues and a tank. Wish me luck.
First playthrough was about 40 hours for me, second was around 60, although I don't remember doing many more sidequests. That should be the minimum for a single-player RPG really, anything shorter and I would feel slightly ripped off. KOTOR was a bit shorter and ME about the same length if I remember correctly.
The main quest choices are, however, somewhat constraining, but I enjoyed the morality system a bit more than in most Bioware games. There were often times when I got annoyed with characters for doing illogical things (*ahem* ALISTAIR *ahem*). I guess they wanted to make a game with no perfect fairy-tale ending, and that's what we got.