I think it depends on what you plan to build in a Matx case. If it's a celeron or p3 system, most anything will work, largely because of the low power/ventilation requirements. If you're talking about a high end AMD XP, hot vid card, etc, then it's more problematical.
I've been toying with the idea of a Matx AMD XP system for a while, and came to the conclusion that I wanted something that will accept a full size 300w ATX PSU and that has good stock ventilation potential. Narrows the field considerably, particularly because low noise is also a concern.
The Inwin v500 fell out because there's no rear fan, The FK 611 is a contender, as is this case-
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Never seen one in person, but it's a LiteOn, probably not junk.
And there's the Superpower 5xx series, here-
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These guys seem a little overpriced, but they have the features, and the mid tower Superpowers I've dealt with were well built, sturdy. These micro-superpowers are hard to find.
Lately I've switched directions a little, trying to get all I want into an even smaller custom enclosure. Now I'm holding out for the Epox EP-8KHM, little brother to the EP-8KHA- anybody seen 'em for sale anywhere?