They are a bit underpowered. They have a smaller cache size and are basically an underpowered PIII. I tried a 800Mhz version and it ran ok for email and such. I even played some DVD's on it. I think the Athlon XP2800 I am using now is a lot better processor. Also VIA does not give much support for their products. They use a really small motherboard (For Mini-ITX) and put too much integrated stuff too close together and it can cause problems with quality of sound and interference, and background noise (Electric Interference).
Via tends to be developing processors at a pace quite a bit behind Intel and AMD. They are working on a new processor, but I have not seen any motherboards or systems built for it yet. IBM is actually working with them on their new processors. At best this is a nitche market for people who want smaller PC's more than powerful PC's.
I think their new processor is like a 800 Mhz Quad Pumped PIII on steroids. I have not seen any reviews on it or a motherboard to run it as of now.
This is my personal opinion.
www.tomshardware.com
Had a review on the new MATX Pent-M motherboards that A-Open is making. On their review they said that this motherboard runs cooler and only uses around 37 watts with an integrated video option, and that it uses about 100 watts with a full blown video card. This means it is easier to cool, and runs quite.