The short answer to your ND question is "no" using and ND filter doesn't give you a more useful image in all conditions, only in very specific ones.. an ND filter regardless of settings may or may not be a good idea...
If your doing my experiment inside, don't use any filters.. and you can stay automatic, the effect will still work.. you can make it even MORE pronounced by playing with the manual settings.. auto focus might not work easily depending on the subject.. but it should figure it out if you center the subject and hold steady..
ND filters just cut the light coming in to your camera, so that means that image WILL be darker. For indoor stuff under normal lighting ND filter will not be useful as the light level is already too low..
To get shallow Depth Of Field (DOF) you need an "open" aperture and shallow focal plane..
Think of the aperture like this.
Its like the pupil in your eye... when its bright outside your pupils closes down, when it gets dark it opens up.. Taking that analogy further, imagine your outside in the bright sunshine, everything is TOO bright and sharp... So you put on some sun glasses.. now, less LIGHT is entering your eye, your pupil opens up a bit and you can see better... an ND filter is like sunglasses for your camera. And just like sunglasses they come in many "darkness" levels.. so an ND 9 is "darker" than an ND 1.
For shallow focal plane on camcorders we achieve that my zooming in on the subject...
Setting max gain to 0 is VERY noticeable indoors.. a typical lit room at night will be almost black without any gain.. which is good and ACCURATE from a control point of view, but might be disappointing from a purely "wow thats pretty" one
But I think all that is just academic, this camera takes amazing images even in full auto mode.
How are you viewing your footage? Have you hooked it up to a big screen 1080 TV directly?
Or are you using the include software to make DVD's?
One thing I did note when I first turned on the HFS100 is the default quality is NOT set to the max..
For basic point and shoot, set White Balance to either indoor\outdoor as appropriate.. Quality to MAX and go for it..
Thanks