Is the Nintendo DS import-friendly

Yes, but this is depending on what you mean as "import-friendly"

Import-friendly, the tax way: (Section 1)
You may have to pay a sum of import tax when it arrives in your country. Check a seller's help section on their policies of deliveries to your home. You may have to pay for for any tax as your door when your parcel arrives.

(For ease of understanding, I will label Nintendo DS software under "Generation 1" and Nintendo DSi software as "Generation 2")
Import-friendly, the computer games way: (Section 2)
The Nintendo DS (Gen 1) can be bought and used in other countries, but please check though the voltage and watt output of your DS charger and your countries standard home power strength before recharging your DS. The electricity may be too strong and could permanently damage your DS if incorrect. The Nintendo DSi (Gen 2) currently (1st Jan 2009) can only be bought from Japanese sellers.

(Section 3)
The Nintendo Ds (Gen 1) games bought from any country should work on any DS console (Gen 1) unless specified.
Many games have WiFi options which will allow multi-player action with other EU, USA, and JP regional games, but this may be not the case for non-Wifi multi-player (e.g. 2-4 DS' play in the same room, no WiFi used). This is usually because of the language encoding for the game.
(For example, JP character encoded alphabet may be different to the USA character encoded alphabet , EU extended character support for many different languages may confuse JP and USA games)
(Section 4)
Software made for Nintendo DSi ONLY consoles (Gen 2) are region locked.
This means if you purchase a new Nintendo DSi from Japan and download DSi software from EU, or USA sources, it will not run on your Japanese DSi unless specified. Same for EU Nintendo DSi consoles and USA Nintendo DSi consoles. The DSi software will only work on consoles of the same region.

(Section 5)
All DS (Gen 1) games made for both Nintendo DS (Gen 1) consoles and Nintendo DSi (Gen 2) consoles from around the world will work on any Nintendo DSi (Gen 2) console unless specified.
The DSi consoles follow the same rule as stated in Section 3.