My post will contain spoilers:

I didn't hate the episode as much as most people. I liked that El went off on her own to figure herself out. Did she need an entire episode dedicated to finding her "sister", probably not. However, Lost Sister was not shot like the rest of the season, so it would have been very distracting going between big city and Hawkins scenes. I liked the introduction of Kali and I liked her band of misfits. It was smart to make Kali and the misfits extremists (they weren't doing petty crimes, they were murders) because it was exactly what El needed to make her run back to Hawkins. I liked that Kali told El that staying or going is always her choice.

If the season was only 8 episodes again, I would have been annoyed with Lost Sister's inclusion but as is, I thought it was inserted at the right time.

I liked the introduction of Kali and I liked her band of misfits

I think that really that was the main purpose of the episode - to show that numbers other than Eleven were out there. Whether to be used as a future direction for subsequent seasons or as a spin off remains to be seen...