Yes, the M3 Simply and R4 (same card, just different skins on the interface) are fully drag & drop compatible, so they'll work with Mac or Linux or anything else with USB Mass Storage drivers. Of the current slot-1 cards, those are the ones I think look nicest. Compatibility with commercial DS games is pretty perfect, though download play for multiplayer or demos is problematic as it is with most DS flash kits, the interface is very skinnable and customizable and they seem to be quite well made and well supported.
The DS-X is also nice, though. Compatibility with commecial software is somewhat better--it has no download play problems form what I've heard--and its interface is even more customizable. The DS-X also has pretty much perfect support for homebrew, which is one area where the M3 Simply/R4 has problems--games or apps that require FAT access (i.e. use external files in addition to the main binary) won't work, though this won't be much of a problem with the new DLDI FAT lib, but for a handful of older apps that might not get updated, you'd be out of luck with an M3 Simply or R4.
The main difference between the two cards is obviously that the M3 and R4 take micro-SD and the DS-X has built in memory. External memory is definitely more flexible in the long run.
However, you have to keep in mind that better slot-1 cards than either of these will definitely be coming out in the future. That's just how the flash card market is. You have to be able to deal with that.
Personally, I think getting a slot-1 kit is a waste. GBA slot kits have the huge, huge added bonus of being able to play GBA stuff. There's so much GBA homebrew out there, it's sick. And of course the commercial games, too. Why turn your back on all of that when GBA slot flash carts are just as easy to use and no more expensive? A GBA slot cart and a NoPass device is the way to go.
...word is bondage...