I have an M3i Zero. Anything you want to know specifically about it?
I will say that there are 2 different firmwares, M3 Sakura and Touckpod, that each have advantages and disadvantages. In short, Sakura is based on Moonshell and has a very nice interface, while Touchpod has better homebrew compatibility. When booting the card, you can switch between the two firmwares, so if you get that pesky homebrew that doesn't work in Sakura, you can boot up Touchpod to use it.
I think both have 100% backup ROM compatibility, though.
Also, M3 Sakura is based on Moonshell 1, so it can't play file formats Moonshell 2 supports. However, Sakura can play Moonshell's DPG videos, but it can also play some Sakura-specific video format, which looks better, seeks faster, and isn't that much bigger in file size.
The two firmwares together is a little over 100 MB, though.
One more thing... DSi doesn't support pressure sensitivity for Colors!... The line will gradually thicken or become opaque as you draw, so you can still draw thin lines, but it isn't exactly the same.
One very last thing... While I normally like them, I've read that Deal Extreme apparently sells fake M3i cards, so don't get it from there.
Any other questions?