I always felt their love came off as Ino's second choice. Then again Kishimoto is horrible at writing romance which is likely why it feels that way. I mean even Hinata / Naruto or Sasuke / Sakura doesn't exactly overflow with romance either, the only pair to do that is Kakashi / Gai. Now that is true romance incarnated.
I'm half half about Ino and Sai due to the nature of Sai. He was legit replacement for Sasuke in the Naruto series until Kishi knew what to do with Sasuke and the overall direction of the series.
Sasuke and Karin would have been way better together than Sasuke and Sakura. Both were obsessed with Sasuke, but Karin was older, had a better reason to love him, and didn't idealize him as much.