Mine is a 1 stop gradient ND. It's dark enough to shoot the sun nicely
for strong sunsets, bright stadium lights, or other similar situations.

I've used it in both horizontal and vertical positions to obtain the
exposure I wanted.

Get the strongest you can find, and it still might not be enough,
that's why people stack multiple filters to make it stronger.

If it is only 1 stop, you might as well not use it, but digitally
brighten your dark scenes 1 stop.

http://digitcamera.tripod.com/#slr

The practical difference is that, if you use a digital SLR, you must use
a circular polarizer.  A linear one will interfere with the autofocus.

There is no difference whatsoever in the effect of the filter.

On the simplest level: A linear polarizer consists of a single element:
a polarizing screen. A circular polarizer has a second element -
something called a quarter-wave plate. It sits between the polarizing
screen and the lens.

The polarizing screen is what gives you the visual effects: blocked
reflections, darker sky, etc. It's the same in both types of
polarizers. The difference between a linear and circular polarizer is
quarter-wave plate exists solely to make sure that your autofocus and
auto-metering systems work properly.

The (simplified) physics behind it: All light waves have an
orientation. A particular light wave coming towards you might be
oriented up/down, or left/right, or diagonally, or whatever. The
polarizing screen can be thought of as a bunch of very narrow parallel
slits, that only let through light waves that have a particular
orientation. (This is not quite true, but it's Good Enough (tm) for
now).

In a room lit by, say, an incandescent light bulb, the light waves are
oriented in all sorts of directions, so a polarizer blocks some, and
lets others through, evenly across the whole scene. The effect:
Everything gets a little darker. In this case, there's not much
difference between a polarizer and a non-graduated neutral density
filter.

However, in an outdoor scene, the light coming from the sky is strongly
polarized: the light waves tend to be all oriented the same way. By
rotating your polarizer to be perpendicular to them, you can block a
lot of them out. This is why the sky gets darker: you're blocking the
light coming from the sky. This is also why you can block reflections:
when skylight is reflected off of water or glass, it keeps its
polarization. The polarizer, if rotated to the right angle, can block
out these reflections.

All this comes with a side effect - after going through the polarizer,
the light is polarized. This means that all the light waves are aligned
the same way, in nice neat rows. This has no effect on the actual image
captrued by film or digital sensors. But it can have an effect on the
TTL metering and autofocus systems in your camera. When the light comes
through your lens, it goes through a beam splitter that sends it in two
different directions: some of it goes to the viewfinder, so that you
can see what you're pointing the camera at, and some of it goes to the
autofocus and metering sensors, so that the camera can figure out what
to do. Some beam splitters don't work well with polarized light: they
may not send enough light over to the sensors (in which case your scene
will end up overexposed, and your autofocus may not work)...or they may
send too much light to the sensors (in which case your scene will be
underexposed).

So, the circular polarizing filter was invented. The quarter-wave plate
"de-polarizes" the light...it scrambles it all up into random
orientations, so that the beam splitter works properly. It's this extra
element that makes circular polarizers cost more.

So...do you need a circular polarizer? Well, I'm no expert on beam
splitters or camera design. It looks like some of them will work fine
with polarized light, others won't. Maybe there are additional factors
that I'm not aware of. It is generally advised that, if you use TTL
metering or autofocus, get a circular polarizer. You could try a linear
polarizer first, and see if it works with your camera, but it's not
always easy to tell if the metering is wrong.

- Darryl