You wouldn't even know it's a spin-off
When I first started watching Toaru Kagaku no Railgun I had no idea it was actually a side-story to Toaru Majutsu no Index, and I still haven't watched that one, as I got hooked on Misaka. Honestly, what isn't to like about her? Super cool, a bit of an airhead, super strong, and likes cute things. It's just win after win with her.
But the characters aren't the only saving grace. Whilst the story has some moments where you are unsure why it's there, it definitely isn't a snooze-fest, and does a good job of tying in previous plot points into the current story.