In my opinion, they're more akin to travelling advertisers, except that in general, they actually give useful information (you can't really promote stuff in forums without giving information) (well, some travelling advertisers also give useful information. Unsurprisingly, they also fare much better at product promotion that those who just scream "Buy my stuff!"). Whether I hate them or not is dependant on one thing alone: if what they're advertising is actually good. If it's good, kudos for them on getting paid to promote something good. If it's crap, shame on them for lying.
To put this in perspective, I would treat them equal to any normal forum-goers, since it's nearly a practical impossibility to differenciate one from a) a secret marketter, b) a member who actually has something useful. Similarly, it's also a practical impossibility to determine the true gender of a person based on purely online interaction, thus it's safer to just treat both genders as equals.
Of cause, there's the non-secret type of marketter, who acts as a person from a company. In this case, the trick is to appear unbiased (example, if your company does something wrong, admit it, and convince readers that something is being done) while providing "insider information" which for most purposes, would probably go through the marketting department to determine exactly how much and what type of information to deliever.
In conclusion: trust no one but yourself for final judgement. Information from others are useful, but take note that occasionally people may deliever inaccurate information, be it intentional or otherwise.