What do you mean? That she is kind or that she is cool?
She has a kinder, more sympathetic feel in the CGI films, but in the 80s series she can be vain and mean. In the end though, she generally cares for her sisters. Overall, I wouldn't say she is "really nice" in this regard. Now if she is cool or not, that's a matter of personal opinion. There have been times I've really disliked Brittany as selfish. While I might not be a huge fan of Brittany, others see her in a different light.
No way! She fell out with Alvin in The Road Chip! Also I have seen her to lose her temper quite a lot so I wouldn't claim her nice since Alvin never loses his temper against anyone because he just likes to have fun which then annoys Brittany.