You may be intentionally doing this to affect the player, but just to provide player input: I HATE mouse inertia and I suspect a lot of people feel the same way. Many people turn off mouse acceleration and filtering in games for this reason, because otherwise the mouse is one of the easiest things to feel changes in. Lag, acceleration, inertia, filtering, etc. may at best make players angry and at worst make them quit.
Maybe I'm wrong, and I certainly don't mean to be rude by saying that, but I think it's important to be aware of.
No worries.
I know where you coming from with this. At times inertia etc. is felt as lag,
if done poorly. Another issue is if you use a mouse in a game the same way as
on a desktop, yet changing its properties away from how the mouse behaves on
the desktop. This is bad practice, no doubt about it. On Windows, for
example, it is always recommended to use Window's own mouse interface to
counter exactly that kind of problem because Windows applies certain filters
on its own to the mouse and makes it function the same across any program,
unless an application by-passes that mechanism, for example, to read the raw
data from the mouse to gain higher precision etc. in which case you are
required to process/filter the mouse data all by yourself.
For a game, however, the situation is somewhat different. If for example you
use the mouse to control a craft, it doesn't make any sense that the craft can
be turned in an instant, and it wouldn't make any sense if the craft would
react to each tiny discontinues motion of your hand or jitter/noise coming
from anywhere else (mouse pad etc.). These kind of high-frequency mouse inputs
needs to be filtered out, and rest assured they are on any modern OS, and need
to be done in games as well unless you use the OSs mouse interface doing the
work for you. But even in that case the mouse may still be too quick, too
discontinuous for your game which requires you, as a game developer, to tailor
the mouse behavior to the task at hand, which is exactly what I did. My
desktop mouse moves way to quick which makes it very uncomfortable to use for
navigating in 3d, yet fine for 2d desktop. Simply scaling the mouse data
doesn't make the problem go away. So for one I wanted to get rid of all the
high-frequency mouse data, yet I also wanted a cool little way to have some
inertia to mimic sort of a craft roaming around.
The filter I've built isn't a fixed one, it allows to set the cutoff frequency
as you please, the coefficients will be computed anew (which is no problem,
because I used a simple IIR filter which is way fast, faster than FIR, yet you
have to watch out for a few more things (blow-up issues etc.)). Not only do I
cut out all the high frequencies (which make the mouse uneasy now and than), I
can also control the inertia because the filter is based on an RC network and
the RC time constant basically control the inertia of the system. Pretty nice,
if you ask me.
Hence, even if I don't need any inertia to mimic a simple craft or something,
the filter does a pretty good job filtering out the high frequencies of the
mouse input, which changes the experience night and day compared to how the
system reacts when the filter is turned off. With the filter applied it feels
rather solid, smooth, natural.
A couple of hours ago I successfully applied the same filter for keyboard
control. The keyboard now experiences the same inertia as the mouse does,
hence, both are in-sync and it all feels quite natural. And, as said above,
even when no inertia is needed, the filter can be adjusted to smooth out the
discontinuous behavior of the keys making the keys essentially behave like a
good solid switch (that's similar like using an extra capacitor for a not so
good switch in electronics). The rise-time of the filter can be made short
enough such that you won't experience any inertia/lag, yet the strongest
discontinuities are gone, which is a huge advantage even for the hardest
hardcore players.
However, basically, you are right. Extra care needs to be taken when tinkering
with well established input controls. The only problem I have with inertia is
if I experience it as lag, i.e. if the inertia doesn't match the gameplay or
something.