I make a great first impression... and little else.

Being yourself such isn't a bad thing (as long as it is not too icky). Keep in mind that people's impression will not stay the same; they rise and fall as they know you more and more.

I'm just like you, to be honest. The main differences here is that I'm usually reserved and tend to keep a certain distance towards new people; thus people see me as quiet and well-behaved. But once I feel they get closer, I just let the nature gush out, since I believe if they're my friends, they'll accept it (as long as it is not too weird or something like that). My nature is that I tend to speak out my opinion or criticise things objectively and honestly no matter what.

And as expected, their impression declined a bit but they still think me as their friend and gladly accept the 'sharp words' when they ask my thoughts about something. And if I joke with sharp words, they will throw back the joke with another sharp word too.

So the point is here: do they treat you differently as their impression changes? If the answer's mostly no, then you have nothing to worry. But if the answer's mostly yes, maybe it's time for some retrospection.

Jam it back in, in the dark.