they run ok, the no speech oldies especially. I get a little slowdown with Sam n Max since its doing full speech and cd music playback. Other titles run flawless.
Its not without irks though. The DS screen resolution is unnatural. All emulated games scale the width down to fit, meaning a little is cropped off of the height. Thankfully the second screen is used as a zoomed view that tracks your stylus... and even who's speaking in most games. So games are still playable.
What's nearly impossible to use is the emulator menu used to set up games. Its currently scaling down the emulator default and is impossible to read and navigate without some prior scummvm experience. It really needs a DS specific menu.
Due to memory constraints it comes with multiple builds so depending on what game you want to play, you must run the right build. You also have to manually patch them all with your specific DLDI.
I have Sam 'n Max, Beneath a Steel Sky and Monkey Island 1 on my n-card at the moment. Build A and B of ScummVM sit at the /root of my card along with all my other homebrew and the games data I have in individual folders (ie:SAMNMAX) inside either the scummvm_A and scummvm_B folder to make it easier to set up the games for each build. As long as the individual game files are kept together they can be put anywhere on the card.
http://wiki.scummvm.org/index.php/Datafiles has the list of required data files. www.scummvm.org also host files, including free full versions of some games.
http://scummvm.drunkencoders.com/ is the ds port homepage. version 10 has not yet (will not?) been ported.