I think I would have to agree with everyone else. The difficulty level of the material (biochem, etc) is easier than it is in undergrad but the sheer volume of material is what is tough. I think it also depends on the school you attend. My school has a systems-based curriculum, which is awesome by the way, and you really get a lot of repetition, so if you don't get something on the first time around, you will probably see it again.
As far as my study habits, it just depends on the day. I typically put in about 4-5 hours a day, but I also work out, watch ER every week, and spend time with my wife and kids on the weekend (they live three hours away).
So medical school is definitely do-able, you just have to balance your time but keep your sanity at the same time!!
Chris