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I can't really answer the "mainline opinion" about Juche, and I'm hopeful that someone else can correct any errors I might have.

Juche is basically just Marxism-leninism applied to the conditions of the DPRK, and with some extensions of the Juche philosophy:

In his early years of revolutionary activities, the leader was well versed in Marxism-Leninism. But he did not confine himself to applying Marxism-Leninism to the Korean revolution but pioneered a new phase of revolutionary theory from a steadfast Juche-based standpoint and resolved the problems arising in the revolutionary practice from a unique angle. The leader discovered the truth of Juche idea in the course of the struggle against bigoted nationalists and bogus Marxists, flunkeyists and dogmatists, while hewing out a new path for the revolution. Finally, he explained the principles of the Juche idea at the Meeting of Leading Personnel of the Young Communist League and the Anti-Imperialist Youth League held at Kalun in June 1930 and put forward a Juche-oriented line for the Korean revolution.

"On the Juche Idea" p. 9/77.

And a little on what that Juche philosophy is:

As the leader said, the Juche idea is based on the philosophical principle that man is the master of everything and decides everything. The Juche idea raised the fundamental question of philosophy by regarding man as the main factor, and elucidated the philosophical principle that man is the master of everything and decides everything.

Ibid p.10

I need to read more from the DPRK, this is basically all I've read. It's quite interesting, though!