I only watched VOY for the first time recently, and all in a short span of time, so my opinion may be different to most.
I thought Blood Fever was a nice episode, and even though it wasn't a full on Borg episode, I liked what it added to the Borg story, and it also set up the de-assimilation of 7 of 9 nicely (even if not intentional).
Scorpion was a great episode, and I think easily the best thing VOY did with the Borg. Maybe the alliance with the humans weakened them, but I liked the lesser of two evils storyline.
Dark Frontier was ok, but not particularly memorable. Even skimming the episode now, I can't remember a lot about it, and I only saw it a month or so ago.
Unimatrix Zero wasn't terrible, but didn't feel like it was worth using the Borg for a big double episode. I thought the concept was a bit silly, and humanized the Borg too much.
And unlike most, I liked Endgame. Again it didn't feel like the Borg aspect was necessary at all, but I thought it was still done ok.
Then we have the minor episodes like Collective and Drone. Collective was pretty bad. Drone was pretty good. It had more elements to the story than just reusing the Borg.
Overall I don't think they were overused, but they weren't used very well. After Scorpion it was downhill. I don't think they were bad episodes, but not as good as they should have been for big double episodes featuring the Borg.