Would you tweet an advert in exchange for free crisps?

Can Twitter be trusted? The number of tweets being published in lieu of hard cash seriously makes me wonder. And if one of the people you follow has suddenly started tweeting about Cracker Crisps, you should be wondering too.

Said crisps are Special K's first foray into the savoury snacks market, and Kellogg's is using a "Tweet shop" in London's Soho to promote the product. The idea is customers "pay" for a packet by tweeting a message about the snack to their friends and followers.

The shop is staffed by women dressed in the red synonymous with the Special K brand. Customers sample their wares in a "try before you buy" area and write their Twitter review. The Special K representatives then check every customer's tweet before handing over a 60p packet of crisps.

So far the tweets have been gushingly positive. Ross Evans tweets: "Mmmm Special K Cracker Crisps, I could eat a whole box and not that many calories #thetweetshop". Which is fair enough, but it jars somewhat with his Twitter bio which reads: "City, Gravy, Beer, Fun".

Transatlantic Blonde made me laugh by living up to the cliché: "Thanks @KelloggsUK! Worked out that a serving of 21 of the new cracker crisps is only 3". Three what? Stones in weight? Million pounds?

Jon Hunt, meanwhile, thinks Cracker Crisps are "the future of snacking" and even posts a photo of the carton-laden shelves in the Tweet shop.

And so on and on. There is not a single negative tweet, which makes me slightly suspicious, particularly as a colleague and I have just tried the salt and vinegar flavour and agree that these are not the descriptions we would use. The crisps aren't bad, but they have that familiar cardboardy background flavour common to low-calorie snacks.

Do the hungry tweeters have to post a positive tweet to earn their freebie? A spokeswoman for the brand says wording for three different tweets is being suggested, but customers are free to write their own: "Tweets are monitored before the Cracker Crisps are handed over. However, anyone can tweet a negative tweet and we cannot stop this."

There are now several firms which specialise in promoting products and services by paying Twitter users in return for positive mentions on the social network, and many companies are encouraging people to "like" them on Facebook by incentivising them with competitions and offers.

Would this kind of marketing convince you to give up 140 characters for a freebie? Does it just make you doubt every tweeted recommendation you read?