I've got fed up searching through my past posts to copy and paste posts in reply to people using DirectInput for keyboard input. So I'll make an entry here, and then just link to it from other topics, and I'll edit this entry to add points.
Thanks to S1CA for keymaps, Input Method Editors and voice control points.
Reasons against using DirectInput for any form of keyboard input:It creates a seperate thread to just read data from the keyboard using raw input (Which you can do yourself with Win32), meaning there's more overhead than just doing it yourself.
No support for keyboard repeat at the rate the user has set in the control pannel - you have to re-invent this yourself. Not too bad for game input, but a pain for GUI-style text input.
No support for capital letters and shifted characters - you have to detect caps lock / shift being held as well as your normal character (As Namethatnobodyelsetook was saying).
No support for caps lock on/off; it's handled by a higher layer than DirectInput, so if someone starts your game with caps lock on, it gets confused.
No support for keymaps other than US-English. A German user of your app pressing Z on their keyboard will show up in your app as a press of DIK_Y.
No support for Input Method Editors (commonly used for languages like Chinese).
No support for accessibility keyboards and things like voice control that require special drivers.
And reasons not to use it for Mouse input:
No support for pointer ballistics (Pointer acceleration, more or less - but more complicated), which makes GUI style apps feel very strange
No support for getting mouse position in pixel coordinates (I.e. GetCursorPos() equivalent)
Much more code to get the same effect as Window Messages.
It's recommended that you DON'T use it by Microsoft; Link:
|Overall, using DirectInput offers no advantages when reading data from mouse or keyboard devices, and the use of DirectInput in these scenarios is discouraged.|
There's one occasion where it's acceptable to use DirectInput for mouse input, and that's if you need high DPI mouse input and you need to support pre-Win2k machines (Where raw input isn't available) - and that's supporting an OS that even Microsoft isn't supporting any longer. There are no other reasons you should ever use DI for mouse or keyboard input.
EDIT: Added mouse input stuff.