OnlyWire has made my life a whole lot easier. I just click a button, type in some information, and my latest blog post or Squidoo lens is submitted to tons of social networking/bookmarking sites. And, unlike similar services, it remembers which sites you're a member of so you don't have to fill it out every time you want to submit something.
This is not to say that it doesn't have it's downsides. For one, I am a member of many more social sites that it does not currently provide this service with. However, they have been adding more sites fairly steadily and I'm confident that they will soon have most of the major sites included. Another problem is that a lot of times some of the submissions are denied. I'm not sure what the cause is of this but it isn't too bothersome.
Overall, though, I think the pros outweigh the cons and it is much more reliable than other sites. Anyone who participates in social marketing will greatly appreciate this convenient little tool.