Not all PPC, just second and third grade PPC services like 7search. They are only fraud, fraud, and... fraud again! 100% artificial and/or incentivized traffic! Ads are distributed through XML feeds to fraudster networks generating massive amount of fraudulent clicks. Stay away. Just ask yourself, who on Earth would perform a search on 7search (and the like) if they need to find something on the Web?! My answer is - nobody that is worth advertising to!
Not to excuse the shady practices of some SEs/ad networks, but a lot of the problem with the lower tier services is that hardly anyone has heard of them, so they don't get enough of the "traffic of good intent" to mask the amount of fraudulent traffic that's out there. OTOH, the big engines/networks are well known and promoted, so they get lots of "well-intentioned" traffic -- enough to alleviate the fraudulent traffic they get in some cases.
I tried some searches on 7search and got reasonable results in most cases. Not quite as good as G, but these were nontrivial queries. (They did much better on trivial queries.)