I'm currently rewatching this on Netflix. I'm in the middle of season 4. I used to love this show aside from the last season & the Kromaggots. Although I didn't remember many of the episodes, on rewatch I still share the same feelings I had which are:
I loved the professor & missed his presence. His personality, older experience & what he brought was never replaced at any point. He really was awesome. I liked the addition of Maggie and wished they had kept the professor as well. I was fine with the idea of 5
The brother was meh. Haven't gotten to the last season yet (but I remember not liking the newbies much or they just weren't as good as the having the real Quinn. so I'll see how I feel this time around)
I still hate the Kromaggots. The idea was good. But they were so OTT evil that it just got to be too much and really pulled the show down.
Overall I really love the concept and it was fun seeing all these different worlds. A reboot would be nice.