I find both 8 and 9 largely watchable because of Capaldi, but very uneven in terms of writing. The arcs in both are rubbish, especially in 9 - despite the latter having better stand-alone eps (and two-parters, no less!). I hate Hell Sent, with a passion.
Yeah, capaldi is always good. but, for me, the Clara/Danny stuff killed Series 8. It had some good episodes, but overall was probably the worst series of NuWho in my opinion. Comparing the seasons by listing episodes I liked:
Series 8:
Deep Breath
Into the Dalek
Time Heist (My favorite episode of the season)
Mummy on the Orient Express
Series 9:
The Magician's Apprentice/The Witch's Familiar
Under the Lake/Before the Flood
The Girl Who Died/The woman who Lived
Sleep No More (borderline, but its decent enough)
Heaven Sent (This season's best episode)/Hell Bent (not that bad, I absolutely loved having the Timelords finally return and while I wish Clara was killed off, as long as she stays gone I don't really care why she's gone)
The Husbands of River Song
Series 9 was a big improvement in my opinion. Even the story I really loathed from series 9 (the Zygon Two parter) didn't piss me off as much as parts of series 8 did. I'm hoping Series 10 ends Capaldi's era well, and surpaces Series 9 in every way. but, even if we just get a comparable number of good episodes, I'll be ok with that.
Back on the main topic, do we know if they're going to do another series of two parters with Series 10? I hope they don't, I prefer individual episodes most of the time even if Series 9 had several good two parters