From my experience, in return for the review you get 2 free copies of the book and a mention.
Honestly, I feel that aside from family and friends of those responsible for the source work, reviews generally go unread by the bulk of the masses. I really think most PR persons are incredibly lazy, and instead of making efforts to think outside of boxes, rely heavily on the old standbys of reviews, interviews, and press releases (ready for copy and pasting among the blogging elite). If the creator responsible is not "a name", then PR persons are compelled to get someone, anyone, to view the work. And then, more often than not, the PR persons pray that such person will do the selling for them.
I realize this sounds bitter. (I've spent far more efforts on promoting indie creators than I have on developing my own works, to very mixed results.) But ultimately, composing review articles is essentially unpaid adcopy work. I think a strong review should be handled along the lines of a high school book report. The habit can be a neat exercise unto itself, as far as writing in general. But there are many snakes in the grass.
All said, if the work/s in question truly strike you (for good or ill), then by all means have a go at formulating your opinion on the matter for others. If only for the sake of trying something new. But be weary of getting locked into it. You do one review, then the publisher or producer may well send you another soon enough. And as I am fond of saying, inundation only suits dry fields.