Not a long review here; just some comments.
These M+ cards are, in practical usage, about the same speed as Type H XD cards. The maximum capacity for them is unchanged: 1 to 2 gB per card. They are shaped the same, look the same, and Olympus claims that they will work in "nearly all" available XD compatible devices.
Subjectively, I am seeing more and more Type M+ and far fewer Type H cards in retail stores for reasonable prices, but functionally they seem like the same item. I am using one 1 gB Type H, one 2 gB Type H, and two 2 gB Type M+; if there is a difference, I cannot detect it.
If you want to check your camera's compatibility with M+ cards, use the Olympus list found here: [...]