Primary attributes

Ah, the primary system. It's the most direct input that voters get into the political process, but is it good for the parties? Is it good for the voters?

I think that the primary system is probably far better than the way things used to be done: it ensures that the will of the public, and not back-room deals, selects who will be the candidates for office.
In a system where party bosses simply select candidates, the primary deciding attributes are party loyalty and service. This encourages dedication to the party, not toeing the line will make it impossible for you to ever advance within politics, but on the converse side, it means that regardless of how the voters feel on an issue, the candidate is bound by the party.
The primary system ensures that the will of the public is foremost in all levels of political contest from the start of the process.

The biggest disadvantage of primaries, in my opinion, is primarily the monetary one, as well as the fact that in large races the primaries can become heated enough that they damage party cohesion-- and the candidates. Look at the beating that Hillary and Obama unleashed on each other in this most recent primary. It would not be unrealistic to say that their efforts to win the nomination may well have made both of them unelectable in the eyes of the voting public, as they probed each others' weaknesses before the Republicans even got a crack at them. Thanks to their own party's muckrakers, the public is all-to-well-aware of the shortcomings of Obama, and Hillary as well, though that's hardly material now.

In addition, party voters tend to be ideolouges, and party bosses often have an eye for strategy and a greater desire, it could be argued, to capture the seat than to maintain strict ideological purity and avoid political heterodoxy.
As a result candidates undergo an embarassing transformation from staunch liberal/conservative into electable moderate. In the modern era of 'gotcha' journalism, and, most importantly, the Internet, it is no longer possible for a politician to tell blue collar UAW-types in Detroit one thing and rich big-city liberals in New York another thing. It's a lesson politicians have consistantly failed to learn, and that is exacerbated by the transformation required between races to capture the nomination and capture the position.