I'm pretty satisfied with how the Mueller investigation unfolded. Primarily because I don't judge something like an investigation by getting some "preferred results," but rather by how impartial and comprehensive it appears to be. I didn't go into the start of this wedded to the idea that Trump colluded with Russia--it seemed fairly obvious that Russian forces did what they could to interfere with the election, but it was never clear that Trump directly engaged with them. But there was plenty of potential danger signs that he did (including the event that led to the installation of a special counsel, his firing of Comey after reportedly trying to order Comey to stop investigating Russia and his potential involvement) that an investigation was warranted.
From what I can tell, as a non-expert, I think that Mueller put in a complete examination and handled his investigation extremely professionally, so I'm happy to accept the results of his report (which still haven't been properly released, but for now I'll accept Barr's synopsis of it--I still want to see the entire thing released).
The idea that unless Trump was indicted or otherwise deemed guilty of collusion the whole investigation was a waste is silly. Investigations into crimes aren't judged by finding guilt--they're judged by getting as close to the truth of the matter as possible. Considering the evidence, papers and people Mueller and his team examined, it's pretty likely that his report gives us that. The whole point of an investigation is that we don't know whether a person is guilty or not, so obviously the result could return with either a probable lack of guilt or a lack of evidence indicating guilt.
From a purely political standpoint, this result is probably the middle result for Democrats. The best result (politically) would be a finding of clear guilt, as it would throw the Trump administration into chaos and could well have convinced reluctant Trump voters to not vote in 2020 (reluctant Trump voters are still exceptionally unlikely to vote for a Democrat because they are still, generally, Republicans). The worst result would have been some wishy-washy result that didn't cleanly indicate guilt but intimated that he might have colluded. That would have emboldened many Democratic voters to put pressure on Congressional Democrats to push for impeachment--which not only has no chance of actually removing Trump as President (since insufficient Republicans in the Senate would vote for removal) but could hurt Democrats' standing when the Mueller report didn't fully indict the President. A result that there's no evidence that Trump was aware of collusion sits between those two things--Pelosi has no pressure to press for indictment (which would be a bad strategic move) but they also don't have a guilty "verdict" to hammer Trump with.