It should be noted that even though some people might think these are overpowered compared to enchants, that they were actually designed to be the "best in class" weapon enchants. That means, that the DK weapon enchants will/should outperform any normal enchant.
To quote Ghostcrawler (a Blizzard Poster) of the Beta WotLK forums:
Ghostcrawler wrote: I think it's fine if the runeforging enchants don't stack with other permanent enchants because the runeforge enchants can be as powerful as they need to be. It's okay if they are the best in their class -- they are offered as a perk of the deathknight. The current ones already scale much better than current enchants.