I have several websites with very high Google Rank. I did this the old-fashioned way - I put content there that people cared about. It happened almost without my noticing, because it wasn't intentional. I was producing content that people cared about, and the high rankings were an unintended side effect.
Now, there's a new breed of people out there. Not so new, I suppose. They think that if they can find high-ranking sites, and put their links there, that they will magically have popular sites. Although they are wrong, they are right enough that they are willing to pay a lot of money for these links. I've written before about what I think of SEO, and some other folks have been even more forceful, or more brief. In short, I have nothing but contempt for artificial SEO.
Which leaves me in a curious position. One of these sites, rcbowen.com, I largely abandoned several years ago, because it was becoming too problematic to maintain multiple websites. But the Google rank remained, because apparently there's still some content there that people care about. including a set of embarrassingly outdated howtos. on various topics that I largely no longer care about. I keep meaning to go update them, but it's never important enough.
Meanwhile, just in the last month, no less than 6 organizations approached me and offered me absurd amounts of money to put their advertising links on these sites. Hmm. They're going to pay me to put links on sites I don't actually care about any more. What's the downside?
Well, the downside is that I disapprove, philosophically, with what they're doing. Astroturfing - the manufacturing of word-of-mouth - feels fraudulent to me. On the other hand, I am also philosophically opposed to unpaid bills. And is it *really* that much different from putting Google ads on this site? Well, except that it pays a LOT more.
I'm curious what you think. But probably not curious enough to change my mind if you think I sold out.

I'll buy you a beer for some links. What brew do you want?
(Note: I need to cleanup a couple of sites first, but I have a few ideas I hope to publish more useful stuff over the summer, on different sites. So I'll plan on buying you at least a sixpack, ok?)
Oh, wait, you wanted opinions, didn't you?
I'm of a somewhat mixed mind. I've avoided putting Google ads on my personal blog - not that it would make anything - but somehow my own personal blog didn't seem to be the place for ads. I may change my mind.
I have a couple of other ideas of useful content for which I've bought memorable domain names that I definitely do plan to put Google ads on as an easy way to make back the hosting charges.
I would certainly consider loads of money for specific ads for specific companies, as long as 1) I didn't believe they were scammers or selling something at unreasonable prices, 2) I didn't believe they were selling something really morally wrong, and (this one is important) 3) I thought there was some relevance between what they're selling and what my content was.
If you personally like something some SEO type is selling, then by all means offer to write them a feature article about it (and be sure to double the price you're getting!). Otherwise, as long as it's not objectionable, it's OK to add ads - as long as they're clearly marked as such.
I guess it depends on whether they want paid links that don't look like adverts or if they will pay for you to put an advert linking to them on your site.
Neither is of much benefit to your users though if the linked item is not relevant to the site content.
There's a major difference between spraying your walls without permission (comment spam, etc.) and paying you to get an add placed on your wall.
I understand people who don't want adds on their walls, but on the other hand I don't think there's anything fundamentally wrong with adds.
The Google rank aspect is IMO just a modern adaptation of the obvious fact it makes most sense to put adds on popular places.
The important thing is that you don't "change" your values just because significant sums of money are involved. Would you like to be one more piece of evidence that "anyone can be bought"? i.e. don't be a hypocrite. Many small hypocrisies eventually enable people and societies to tolerate large ones as well.
On the other hand, I understand that "new data" can cause a person to re-evaluate a previously-held opinion. Just do your best to understand whether you've changed your general beliefs on the subject or whether you're just suspending them when it benefits you to do so.
If you believe Ads are appropriate at all, and it sounds like you do, I also agree that a relevant question is whether viewers of those pages would consider the ads relevant. Again, "do unto others what you would have them do unto you."
Having been someone who was philosophically opposed to text link advertising, and then when the stars of "absurd amounts of money" and "need to pay the mortgage" aligned actually ran ads, I think I can weigh in on this subject.
I do not regret running the ads, since at the time the amount I made a month actually covered out mortgage payment which was a great gift. On the other hand the damage it did to my psyche and my site's pagerank are things that can never be repaired.
Google penalizes sites that run text link advertising heavily, which I think is hypocritical since the "paid results" on search pages are text link advertising but I digress.
The bottom line is no ones opinion matters but yours and if you are willing to incur the PR hit (my site went from 8 to 6 in a year) and feel the money would make a real positive change in your life then you should go for it.
Oh and I hope you do realize that as soon as Google lowers your PR the advertisers will try to force you to take less money. After a year, they tried to move the amount they paid me from 850 to 300 a month.
Happily I was no longer in a place that I needed to continue with the advertising, so I told them to take a long walk off a short pier.