I have no problems with the seat position relative to the handle bars. It's very comforable for me, no aches or pains so far. I also own a Giant road bike and this feels just a comfortable to me. The front suspension helps smooth the ride as well.

The bike comes in three sizes for both men and women. The fit my LBS did showed that I was between a medium and large size. I rode both of them but thought the medium size felt better. So you'll want to try a couple of sizes to find which one fits best. You'll definately want to get a good fit done before you're test ride to get the seat high enough so that you can get the maximum efficiency from your pedaling.

I haven't done much riding on steep grades so I pretty much only use the top 4-5 gears when I ride. Shifting is very easy and smooth. I've found it better to stop pedaling between shifts. I don't know if that is the proper technique for internal hubs or not but it's much smoother when I do that.

The frame seems very strong and should be able to hold quite a load.

I hope some of this was helpful. The best way to determine if this is the right bike for you is to get a test ride on one and see how it fits you.