If you have the freedom to write what you really think, an honest review type/situation, then I don't see anything wrong with it. You've got a hunch, if you have time, maybe you can follow up on it. Dig a little deeper and if you end up uncovering something then you're a public service. And of course they've got to pay you up front. :) haha
If you can take that approach I can't see any wrong in it.