I have used the last two ubuntu LTS server releases for production dedicated servers. Never had a problem. Its very good stuff for mainstream production systems.<p>I came to ubuntu from FreeBSD. One of the many things I liked about BSD systems is the secure-by-default attitude. Ubuntu server comes close to this in a default install.
As much as I fear for the direction Ubuntu heads in terms of "free software'" and commercialization sometimes, I really admire their release schedule: Consistent, regular releases, with an overlaid schedule that emphasizes reliability and robustness. The best of both worlds!