There are quite a lot of services out there. And they vary widely not only in price but in quality.
You might want to do a mix. One thing Google's latest algorithm does is judge the mix of guest posting. That is, if you do a blog tour that hits only premium sites, or only sites in a certain circle (which would happen with a service), that triggers the "unnatural promotional behavior" red flag, and the posts get lower rankings in search.
Not that it's going to do much harm; you still get the direct promotional aspect of being in front of so many eyes.
You might do some searches on "virtual book tour" and "blog tour." Also, maybe go over to kindleboards.com and check out the Writer's Cafe (it's a sub-forum of the Book Bazaar forum) where there are undoubtedly LOTS of people who have been doing guest posting and book tours. Also a few people who run sites which feature an indie book a day or do interviews.
Personally, I would just start collecting blogs I wanted to appear in -- maybe start an RSS collection so I could read through them every so often, see if any are looking for guest blogs, or do any kind of guest blogging at all -- and start querying bloggers. That way your guest posting is targeted to the right audience. And your backlinks will come from non-spammy sites.
One other thing I did (which I never followed through on, so I can't tell you how well it worked): I started interviewing authors for my own blog, once a week. My original intention was to attract their readers to my blog, and also to build relationships with the writers for later. My blog is not really suited to traditional guest posting, so interviews were a way to keep it in line with what I write about, as well as provide consistent quality content to the readers.
What I INTENDED to do, and never got around to, was to start querying established names in my genre to interview. Most of them would not be so interested in mutual guest posting, but they might be interested in a quick interview, and that would provide higher-interest material for my readers. And I did figure that I would increase the chances of places I could guest post to.