What is it about Spike that makes him such a love-it-or-hate-it character? I think it basically comes down to whether you judge him more by his behavior in Spike-centric episodes or by his behavior in other episodes where he appears, mostly Twilight-centric ones.
In non-Spike-centric episodes, his character revolves on supporting Twilight, and (especially early on) the fact that he’s a “baby” tends to be emphasized. In Spike-centric episodes, he’s a complete jerk. The Mane Six also get their flaws put on display when they take center stage, of course, but as a general rule either they learn their lessons and move on, or (particularly in the case of Pinkie Pie and to a lesser extent Fluttershy) their flaws are harmful only to themselves. Meanwhile, Spike is repeatedly depicted to be self-centered and greedy in a way that hurts both himself and others, so increasingly (if you focus on those episodes as the ones that define him) it starts to look like he’s wilfully not learning his lesson.
So I think that’s probably why Spike has lots of detractors and lots of defenders, but not a lot of people in between.