Hello all!
I thought I would pop into this thread - SUPER fascinating discussion - to share my experiences with @Trevor Mauch and Carrot, specifically in terms of conversion rate.
This is our monthly, agency-wide conversion rate data. It includes trial clients (who start up free in their first month to test their markets), existing, long-term clients, as well as any existing landing page data from before we began working with clients (assuming they had conversion codes in place), for all keywords, over all time.
It's worth noting, to answer @Sean Dolan's valid point about brand terms above, that this is from adwords traffic, including all match types, with almost no brand-specific search queries (which are incredibly low volume). Over the course of a year this accounts for tens of thousands of clicks, hundreds of conversions, etc.
("Landing page engine" above are specific design tests we're doing in-house; "other landing pages" is specific anything non-carrot, including custom sites.)
We have yet to find anything available out-of-the-box that converts better than Carrot - and believe me, I looked. I'm a designer by trade, and I have always thought Carrot sites were pretty ugly...but the fact is, these types of sites simply convert better and perform better than the average wordpress template (typically, I think, because those templates actually look too polished/corporate, though that's just a theory).