I'd love to see a poll of what end users think - non-SEOs, just regular schmoes who may or may not fully understand what a search engine is. Do people feel Google is improving their user experience? Because Google doesn't care what we webmasters think, as we're not who the SERPs are intended to serve.
For years, Google has benefited from people not realizing they had other search options that might give them better results. But it seems to me the typical user is getting savvier - if Google doesn't give them what they want, then most try a site specific search ("Hmm, that's not it - I know, I'll check LifeHacker") and a few will try another search engine. It seems to me that if Google wants to maintain dominance in search, they're going to have to provide users what they want, no matter how that affects any other concerns Google has. If they're doing that, then Penguin is a success. But if not - and honestly, I think it sucks on most queries I've tried lately - then Penguin's got some bugs.