Paid to Post?

I've recently joined a couple of sites that will connect you to advertisers who will pay you to write a blog post and basically advertise for them.  They are supposed to be products or services that will compliment the content of your blog.  So far I have had leads from a couple of advertisers and the ones that have accepted me are not really things that I am interested in writing about.  Simply because I don't really have any real practical knowledge on the topics.

So what do y'all think about this concept?  Lot's of blogs do this, I just don't want to turn my blog into something that is not mine.

Should I start a different blog for this kind of thing?  Have a personal blog and a "professional" blog?

I would love to hear your thoughts.