It goes everywhere I go! Personally, the weight's not an issue, and it fits nicely in my purse. Nothing else makes such a good personal assistant. I use it to schedule my day and take notes when something comes to mind. I generally try to avoid using my iPhone for mundane tasks unless whatever I'm doing requires a data connection (GPS navigation, a quick web search, video streaming) to preserve battery life.
Not to say that I don't love my phone to death, because I do, but the device of choice on the go is definitely the iPad 2. =)