Of course it's partisan on Brexit (as is The Economist, btw) and of course its general direction has a leftist slant (and why should it not?)
That said, I think it's one of the few remaining quality papers in the UK, which provides overall accurate reporting.
What makes me gag sometimes is some of the writers in The Opinion section. But that's fine too. If it's too crack-pottery I'm alsways free to ignore it.
Their Long Read section[1], though, is fantastic long form journalism and that alone justifies my yearly donation.
[1] https://www.theguardian.com/news/series/the-long-read
EDIT : Slight clarification