On the whole, it changes people's perceptions just to hear the idea. But that said, in my experience at least and in that of others I hear about, there's a tendency out there to consider it a "phase" or a "confusion", and sometimes hope that a trans individual will let go of the idea. So until they act on it, most people will give them at least some kind of room and be at most just disapproving or ambivalent.
Seems to me almost everyone you tell will look at you slightly differently, most will accept the idea more than will accept physical changes. And some will not look down on you at all for either.
For those whose lives are directly affected by a person's transition I would find their concern understandable - it will push them into considering things they probably hadn't before thought they would have to consider, if this was not told to them earlier. For those whose lives are not directly affected, naturally they are going to assume something is wrong with you - and in a sense of course it is, or it wouldn't be considered a medical condition requiring treatment - so there's that. Might just be concern for your welfare, concern you might be making a cry for help, concern you've been influenced by something that may not actually be "you" etc. and from what I see, the response is often not dissimilar from people's reactions when they hear a friend or family member once contemplated suicide or reckless behavior - they don't know how to handle it and they think you might be having some sort of unrelated crisis.
There are some obviously who when they hear the idea, have already decided beforehand they're dead against the idea or look down upon it. Could be for religious, ideological or practical reasons. I know a few who look down not on transsexuals, but on the surgery/HRT itself, for what they think of as practical reasons - such as the cost, the risks, the post-operative suicide rates and things like that. Often all of these reasons come from a well-meaning place in the person's own heart, as if they are trying to warn you away from making a great mistake. It's rare I find genuine malice anywhere toward trans people when someone comes out to them. Usually it's just pity, or concern, or their idea of trying to do you a favor and make you avoid what they think is a terrible mistake. Occasionally I see the disapproving parent who is disappointed in the child for doing it as well. And then there's those who see it as bad for society at large to entertain any ideas like it.
It's very much like admitting crippling depression, attempted suicide and the like - you're admitting to a mental malaise they probably don't have nor understand, nor have the ability to negotiate, but clearly has some health negatives associated. That immediately puts you in the category of 'mentally compromised' in a sense to them, and it seems to be that they generally look down upon when they first hear it. The most common response from disapproving folks I see around is "trans people are mentally ill" rather than evil, or stupid, or anything like that, which seems to back up this observation. They think we need help, but tend to think transition won't help us. They usually have no idea what will help us, though, because they're nearly always uneducated on exactly what our condition is.
