Lots of the comments are a bit outdated.
I sell them regularly. Typically they are not meant for entry level users,
Cooling is no longer an issue with them, but the main limitation to them is expandability.
Since sound, network and even graphics is available onboard on most models, you rarely need more than 1 PCI slot. You are limited to 2 or 3 hard drives and 1 optical, but most people never exceed that limitation.
The build quality is extremely good. You will struggle to find a better made PC anywhere. They simply are not available in our price competitive retail stores. They are completely silent with the right internals and provide the same performance as a normal PC, because apart from the chassis and the motherboard, the components are identical.
The only graphics cards that don;t fit are the extremely large top-end cards like the 1950XTX and the 8800GTX. Believe it or not but a 8800GTS fits.
If you like, Private message me and I can give you a quote on one.