I'm going to do a generalized sort of response -- I wish more people who disliked the label would have commented because I suspected that people who DO support it would be more likely to get up in arms over its usage. And that's fair: obviously many transgender individuals have reason to feel very strongly about the term cisgender, the terms that describe them, and any of the relating terms.
I have a couple different things to address that I saw in multiple posts, so I'll go ahead and not address anything individually, but as a whole...
First off, just because you don't use it negatively doesn't mean that no one else does--or that it's "okay" that a few people use the term negatively since most people don't. I'm going to be blunt, knowing that perhaps my situation is rare, but still valid: the only people I know who use the term "cisgender" are people in my campus's Gay-Straight alliance. They largely consist of GLBTQ individuals and saying that it's "gay-straight" alliance isn't entirely the truth. Largely, it is a gay/gay/more gay clique. Although I identify as pansexual, I do not actively participate in the group for this reason. I feel that many of the individuals in this group represent, fairly, a large number of young GLBTQ people. These people, gay, straight, "cis" and trans gender, all use the term "cisgender" in an extremely rude, and derogatory way. The ONLY time they find reason to use the word "cisgender" is when telling us "non-trans" folks that we "don't get it" and "will never get it." Also basically that we are heartless, unfeeling aliens--And frankly, I've gotten this same impression from a few people who posted in this thread.
Second, if you want to argue that non transgender people will never understand transgender people because no one HUMAN can have the exact same life experience as another HUMAN, then I will accept the arguement that "I will never understand." You're right: I can't live your life and I will never fully understand what has happened or is happening to you. But in that regard, no one will ever understand you--not even another transgender person. Beyond this, by saying: "YOU WILL NEVER UNDERSTAND ME!" you are ENCOURAGING "non-transgender" people NOT to understand you. When I hear a transgender person yell at me that I "have it so easy" and that I'll "never understand" it makes me want to brush that person off and disregard them completely. If you refuse to connect to me, then I will refuse to connect to you, empathize with you, or even offer you my pity for your situation--believe it or not, the rest of the world has problems too.
Also, I take it personally when it's directed personally at me. I'm offended by the term when it's used offensively. To say that it's "just a term" is basically saying; "Sticks and stones can break my bones but words can never hurt me!" Are we children in that we still need to make this argument to justify cruelty in adults? We shouldn't have to defend ourselves and rationalize other people referring to us in derogatory manners: people should recognize and respect each other.
Finally... I disagree with people who think that labels are "normal" or "necessary." There is a constant struggle in every minority community--black, gay, trans*, and everything in between, simply to be treated normally and equal. By insisting on labeling everything, we simply widen the gap between ourselves. So no, I do not think that it is okay to label people who are sexually attracted to the opposite gender or people who identify as the gender they were born as or people who are brown or people who think that they're actually a cat in a human's body or people who were born the wrong gender or people who just love everyone. It's not fair and it drives me crazy how obsessed with labels people are--and this is no exception. I suppose from now on, I ought to just start being the person that identifies as "human." People will be annoyed by it, but maybe eventually someone will catch on.