Let's compare/contrast the historical veracity of information released by Assange/WikiLeaks with that of any US intelligence service, shall we?
Anyone who claims that the Podesta emails were not real is delusional. There's no real dispute over that.
Anyone who believes that Assange isn't biased against Clinton is also delusional. He also shows a disturbingly willful blindness to find any problems with the state of civil liberties and human rights in Russia—which, again, is not really subject to controversy.
Anyone who believes that Assange can be certain about the origin of the Podesta emails doesn't understand chain of custody. His de facto imprisonment in the Peruvian Embassy makes it physically impossible for him to objectively, empirically verify any claims of provenance. If this were evidence for the courts, he wouldn't be allowed to testify as to the provenance of the emails.
Anyone who has examined the pattern of overt and covert activities as already detailed by public domain sources that has been judged with a high or a moderate level to confidence to originate from the Russian state would be foolish to deny that there isn't a strong preponderance of evidence that yes, Russia conducted an anti-Clinton (dis)information campaign.
On the record print and TV interviews with avowed state-paid Russian trolls who profess a strong preference for Trump is probitive of a classic old-school dezinformatsiya effort. It's something that both sides used frequently in the Cold War. RT's overt anti-Clinton editorial slant is obvious, but in itself only contributory, not probitive. Assange's frequent appearances on the channel are evidence of nothing more than a bit of narcissism on his part.
The fact that the APT28 modus operandi is consistent with well-documented spying activities against the Bundestag as well as the TV5 cyber-attack is one plank. The fact that APT28 code was almost exclusively developed in a Russian language build environment, in the Moscow time zone is damning. The fact that that they used of bit.ly as an URL-obfuscator—and then committed a rooky OPSEC slip-up that allowed investigators to see what other individuals were targeted by the same account—is compelling. The fact that APT28 source has been found in the wild doesn't diminish the likelihood that this particular use of it originated from the Russian state. The use of encryption keys and certs pretty much makes it impossible for third parties to use the software without significant—and obvious—re-engineering. There is no evidence of such changes. In fact, at least one cert used in the Bundestag hacks was re-used in this effort.
The evidence suggesting that Guccifer 2.0 is almost certainly not Romanian, and is probably a Russian speaker, is not probitive, but it's strongly contributory to a conclusion that the account is a sock puppet, probably linked to a Russian source.
The USA intelligence community lacks credibility. It has relied far too much on its own much-sullied authority to make its arguments. But its credibility is laughable, and its patent insincerity and systematic dishonesty is demonstrated by a mountain of evidence. The fact that their assertions are consistent with open-source evidence indicates, however, that they're not lying about everything—this time. That does nothing to diminish the fact that they're driving a clear agenda, possibly because they don't trust Donald Trump and they feel he's compromised, or at least willing to put personal interest before national interest.
Conclusion: It's not necessary to believe the CIA/NSA/FBI to conclude that there is a concerted Russian effort to subvert the integrity of key aspects of American democratic institutions, including the US Presidential election. The Russian state has motive, means, opportunity and there is sufficient evidence to suggest that, in absence of any more compelling explanation, they have probably been at it for quite some time. Did they 'hack the election'? No. Did they sway it? They certainly put a lot of time and resources into the effort. Did they change the outcome? Probably not. The single event that correlates most closely with an actual swing in the electorate is James Comey's letter to Congress concerning the Weiner laptop. Did they help swing it? Almost certainly, yes. There's a compelling argument to be made that if countless sources—with Russian prominent among them—hadn't worked so hard to poison the Clinton well, the Comey announcement wouldn't have been so decisive.