I think it is a very, very good and clever game. Its certainly one of them that you can really get involved with, and the more you do so, the more you get out of it.
Sure, you could complete it in 6 hours, but I'd guess that just going through the main storyline and nothing much more. There are so many little side missions, so much humour, its fantastic.
In terms of how good or evil you are, there's ample opportunity to do evil missions. Also, the level at which you personally interact with all of the people will heavily determine this - not just charging the highest rent and bumping a few people off in the missions, you really need to do all of the personal interactions.
If you're looking for a fun, less intense RPG that you can get a good 50 hours or more out of, then buy it, and put the full effort in to it.
If you're only going to bez through the main story and then rate it negatively because its not that good - then you deserve to be disappointed. These games are fantastic because it makes you put the effort in. I want a long game that last me for weekends on end, and then i'll get distracted by another game for a few weeks, and then go back to it a few months later to finish it off.
You get out of this game the effort that you put in to it. Its not all handed on a plate for you
There's also a really good twist at 'the end' of the game which I certainly wasnt expecting.