No, no and NO
Far too much spin, gloss and misdirection - even in the response from the Phorm tech guys who STILL miss answering the direct questions - along with the charmingly naive assumption common to all purveyors of this type of 'service' which assumes adverts are somehow useful to everyone - but of course the ability to provide this 'service' is of MUCH greater use to Phorm, which exists only because the web has exploitable users. (I believe the warm and friendly term used was "monetise")
When I browse a site it's for a specific purpose and if a site doesn't cough up what I'm looking for then I simply look at another - generally easily found with any search engine. This whole experience is made annoying by the visual noise of ANY form of advertising so as a result, they're ALL disabled here. I don't WANT to see advertising, PERIOD.
I also dislike intensely the assumption that I'm a retard who requires "targeted advertising" <spits> because plainly, I am incapable of just looking about and deciding for myself. It's far too much like that sodding MS Office paper clip popping up and asking if I want help with writing a letter. Sure, go right ahead: write the whole damn thing for me...
It's completely arrogant and plain WRONG to be opted-in to this 'service' as the default choice: how dare you assume that I'll find useful what you provide, without even bothering to ask me first (and no; undocumented analysis of MY data doesn't count). As others have pointed out, to remain opted out I'd be REQUIRED to keep the Phorm cookie. No, not good enough; this is just shabby thinking. Further, since users who have opted in to this 'service' are helping Phorm make money it only seems fair that these users are paid for the data Phorm is harvesting. But of course it HAS to be opted-in as a default: who would (effectively) tick a box marked 'yes please - pester me with more adverts I've not asked for. The stuff I see on the streets, transport, TV and in newspapers just isn't enough'. The only acceptable form of default action is that my browsing habits are examined by nothing and no-one - unless I request it specifically. (law enforcement actions with the ISP and gubbinment snooping excepted...)
The privacy implications are legion and no amount of smooth talk will convince me that it's anything other than targeted snooping of hog-tied users' data with the sole purpose of making money. And thanks, but no thanks - I've been around long enough to recognize a phishing attempt whether it's in a mail or on a site. It's not exactly rocket science, is it...
The only difference between spam and "targeted advertising" is that spam tries to sell snake-oil or fakes and a site visit would likely attempt to infect Windows with something nasty, and "targeted advertising" tries to sell something legal from a trusted site.
Curiously, while it's well accepted that addresses used for spam are gathered by underhand or nefarious hoovering techniques, we're expected to accept that because Phorm announce it would hoover details sufficient to achieve exactly the same thing as spam (unsolicited information) - it's now supposed to be a Good Thing.
In either case any advertising received is still going to be an unrequested, hit-n-miss opportunistic attempt at getting me to part with my money.