Generally, I have to mostly agree with those who've posted before me, but I'd use Debian GNU/Linux for something that I absotively posilutely have to rely upon. The last HTTP/SMTP/et cetera server I had to take out of service ran for just a tad over five years with the only system downtime being during kernel upgrades, and the only server downtimes during software upgrades.
I've been very happy with Debian for the past nine years or so, and I earn 100% of my living with the various Debian machines I've got scattered around the country, including the workstation I'm on right now.
