They were fun... except for when the writers decided they needed Drama! in the relationship. The character archetypes of chaotic, slovenly slacker and super organised go-getter were there from the beginning, but the writers really played heavily on them to sow division in the relationship.
What I hated the most about the episode with their break-up (and there were a lot of things) was all that "I miss my friend" crap as the justification for the break-up, that they would then stop fighting. I thought they fought a lot more when they were just friends, including an episode late in season 1 that included a screaming match where they wound up shaking their butts at each other. (I can't remember why they were so mad, though I think it was because Jess was telling him how stupid he was to reunite with Caroline.) It was clear that the writers just broke them up because they didn't feel like writing them as a couple anymore, not for any actual thought-out storyline decisions, so after reading comments from the showrunners about how they just wanted everyone to be a bunch of wacky single friends (a complete reversal of a few months earlier), I decided I didn't feel like watching for several years. I had been burned in the past continuing to watch shows that both broke up couples I liked and (usually before that) stopped being as funny as they had, so I took it out on NG.
I haven't seen nearly every episode of The Parkers, but Nikki and Professor Oglevee were definitely a couple that I didn't like. The people in charge clearly thought their ending with the two getting married was something everyone wanted, but that was not the reaction I saw at all from forums at the time it aired. It had been nice to see Nikki, who seemed like a nice, smart woman aside from her stalking of the professor (not a small problem), finally come to her senses, move on and find someone who truly appreciated her. Although it was understandable that Professor Oglevee was frightened by her constant strong advances, he seemed like a conceited jerk toward most people most of the time, so I didn't get why she was so into him. Once in a while, they would throw in something as a hint that he "loved" her, but I didn't buy that it was more than physical; and he would usually turn to her when he was at a low point, then reject her. The finale had two major problems: it rewarded her for her stalking and ignored that the main reason he didn't completely reject her advances over the years was because he delighted in being cruel to her (i.e. he's a jerk). And they were supposed to be a happily ever after? That was definitely a "didn't last five minutes after the finale" couple--I even mentioned that to someone on a forum at the time to cheer them up.