EMDs have the strongest influence. If the keyword match is not "exact" however, the influence of the domain in the equation, and the corresponding rank in the search, drops off quickly. Example of an EMD: "houstondentist.com."
PMDs have less of an effect, and I would go as far to say that they are only of significant value in searches that aren't really competitive. An example of a PMD would be "fourstarhoustondentist.com" where "Four Star" would typically be the business name or brand.
Well, the effect of both of these types of domains has apparently been downgraded by Google recently.
Here's an excerpt from a Moz post
Google's "Multi-Week" Algorithm Update - Moz: In my post on the June 25th temperature spike, I reported a noticeable single-day drop in partial-match domain (PMD) influence. That post happened very early in the multi-week update, so let's look at the PMD influence data across a 30-day time period that includes all of the high-temperature days:
While there was a lot of movement during this period, you can see that PMDs recovered some of their initial losses around July 4th. The overall trend is downward, but the June 25th drop doesn't appear to have been permanent.
It's interesting to note, even if not directly relevant to this analysis, that the long-term trend for PMD influence in our data is still decidedly downward. Here's a graph back to the beginning of 2013:
Because PMDs are more easily obtained than EMDs (who else would want fourstarhoustondentist.com?) they are more widely used and tend to be of lower quality. A spammer can buy a bunch, and in theory get some long tail traffic. I think the trend will continue to downgrade the effect of these domains, particularly on sites that lack the other signals of a quality site.
My own experience is that EMDs I am watching actually got a boost and the beginning of July, but have settled out to about where they had been before. The same Moz article shows a downward decline in ranking for these domains over time as well, which is not surprising. Any SEO tactic that Google knows about will be penalized. It will be penalized, however, in a way that is weighted towards the success of big sites and brands, and to the failure of smaller sites and businesses.