@laia: I still don't have my C504, therefore I can only summarize the feedback of other users and the Schenker support. ;)
The HDD caddy has a production fault which causes the laptop body to "bend". Therefore it doesn't stand flat on a table. Schenker solved the problem by replacing the HDD caddy with DVD drive for free.
Some users still got a bent C504 (with a DVD drive and a HDD caddy extra), but others fixed the problem by screwing off backplate and tighten the screws.
The other big topic is the keyboard. Many C504s have a wave in the keyboard (some keys are higher than others). GIGABYTE confirmed the problem but they cannot fix it because it is a general design problem. The Schenker guys in the assembly try to reduce this wave but you may still have a slight difference between the keys. Some users posted a few pictures of it.
The keyboard wave is only a blemish and doesn't affect the functionality.
As with the bent body, some user say that they have a completely even keyboard. Others could fix it by adjust one or two screws inside the laptop.
Looks like you need some luck to get a good-looking C504 :(
Apart from this, the display is great (nice colours and a good brightness (over 300 cd/m²)*). But, due the slim body and the hugh power, the laptop will get hot and relatively loud (max 53,2 dB)* if you play or work hard with it. On the other hand, user report that it is almost silent during office use.
If you want more information, you may search for "Gigabyte P35G v2", which is equivalent to the C504, but you can't configure it.
I hope I could help you a little bit and sorry for spelling mistakes, it's late ;D
*Review and test by
http://www.hardwareluxx.de/index.php/artikel/hardware/notebooks/32379-schenker-technologies-xmg-c404-und-c504-im-doppeltest.html