In comparison to what other forum software? I know that since all them years ago since I moved from Proboards my forum has grown an incomprehensible amount - due to the lack of advertisements, the own domain, faster loading speeds and obvious freedom and vastly improved software.
I'm not entirely sure what it is you're attempting to get out of this question however - the chances of you seeing a decrease in activity as a result of moving to IPB is highly unlikely.
In terms of what I don't like about IPB - at the moment it has to be IP Content, I would love nothing other than a clean and slick homepage, however not being a particuarly talented web designer this is made quite difficult unless you have a lot of time experimenting with the particular application. Other than that there is probably absolutely nothing that I have a problem with. The beauty of the software is that if there's something you want, with a little bit of knowledge on the coding and software it is easy to add, and there is a fair sized modding community here that allows you to get interesting addons that you might not be able to design yourself.