Well, there are a number of issues to consider here, some of which are ethical and some are more about not pissing off Google.
It is also a very different situation depending on whether you are considering a one off or working with a large broker type operation. I am assuming you are considering these reviews as a means of generating links for search engine rankings.
Selling a single review is a very different thing from having a blog with 50% paid posts. It's Cons and questions would revolve around whether and how you let your readership know that you were compensated for the review and how they react to that. (unless of course you sell links to some devious Google employee).
Bulk buying or bulk selling of blog posts is an altogether different issue. Bulk link buying or link selling will likely leave a footprint that a search engine can detect. If Google, decides that paying for blog posts is an attempt to deceive then the risk is getting caught.
If they decide that you are buying or selling links with the aim of passing PageRank and manipulating results then you can expect penalties.
Of course, they have to catch you first.
The pros are that paying for blog post links seems to work, at least for some. (that is not a comment on any individual service or any bulk service at all).
Taking short term decisions based on what works right now can be very dangerous though. IMHO, it is dangerous and ill advised.
Google has caught up with paid link systems it doesn't like before and a lot of sites get caught in the process.
