It's very individual and depends on whether you are a student, regular joe, a creative or a photographer. For some people the iPad is gods gift but others find not much use and it ends up being a disused overpriced surfing tablet.
For me. It's my main machine and have put me off buying a Macbook Pro since the iPad is perfect for 80% of my computing needs.
Yeah, between my 11in MBA and iPhone 4s, I don't see an iPad fitting in anywhere. If I had a 15in or 17in MBP (maybe a 13in), then maybe an iPad could find some usage.
There's definitely a lot of crossover between an iPhone and a Macbook.