As someone who works on tasks reported on the Wikipedia conflict of interest noticeboard, I've been dealing with the paid editing problem for some time. Wikipedia is worth some effort to preserve; it is, after all, the highest rated non commercial site.

Most of the conflict of interest cases are simple promotion of a band/company/product/self by one person, usually someone with no other editing interests. Some are detected automatically by programs which check new Wikipedia content against the web for duplicates. Those are checked for notability, and many are deleted because they don't meet Wikipedia's criteria for inclusion. A few years ago, this was mostly a problem with garage bands; now it's a problem with garage startups. Those are easy to deal with.

Then there are subjects where there's unquestioned notability, and the subject of the article is trying to look good. Those usually involve content that reads like a sales brochure. Sometimes it's well-cited, but to weak sources. A typical example involves a company trying to list every flavor in which their product is available. The usual response to this is to delete promotional language and unnecessary product detail. The result can be a cold but well-cited article about the company's financial history. For individuals, resume inflation can be a problem. The requirement for a reliable source usually deals with that. Wikipedia, remember, is about "verifiability, not truth".

Promotional editing can backfire. Badly. When promotional editing is found, standard procedure is to look in Google for other information about the subject. If this turns up clear negative info, it will probably go in the article and stay there. Criminal convictions, regulatory actions, accident reports, investigative reporting, and criticism by consumer groups reported in the mainstream press tends to displace PR-type material. If the person or company has done something really bad, their article is likely to focus on that. This can result in frantic efforts by a company to make the "bad stuff" go away.

I was involved with the "Banc de Binary" mess, where the company was desperately trying to remove their legal problems. (The SEC and CFTC had booted them out of the US for a long list of blatant securities-law violations. Banc de Binary is not a bank, or a broker; they're an online casino set up to look like a brokerage. They're the "house"; if you win, they lose, and they set the odds.) Banc de Binary at one point offered $10,000 on eLance for someone who could "fix" the article their way. That was noticed and got press coverage. After much discussion on Wikipedia message boards, the article ended up rather negative on the company, all their accounts were blocked, and many people have that article on their watch list in case somebody tries something.

That's unusual; it seldom gets that bad.

That's the blatant stuff. More subtle spin is hard to detect. Minor changes to obscure medical or technical articles to make some product look good are tough to deal with, because they require subject matter expertise. This is a growing problem. But the usual procedures work; if only vendor X is mentioned, a search for a competitor may turn up vendor Y, which can be added to the article.