Right now I have a MacBook Pro and a 24" Apple Cinema Display, and two iPods (Touch and Nano). It has worked great for me since I can still take the laptop with me when I need it, or use it elsewhere in the apartment and still have a big screen when I need to do work (and cartography is painful without a lot of desktop space). I had planned on buying the iPad when it came out in Canada just because it looked really bloody cool. Now after reading reviews and peoples impressions of it I plan on holding off. That and the fact that summer is almost here and that I tend to ignore my computer during this time of year means there is no rush since it won't get much use right away.
But in September when my girlfriend goes back to school she is going to need a laptop to replace hers, which is almost dead. I had planned on getting a Mac Mini and hooking that up to the ACD so she could have access to to the MBP whenever she needed it and I would still have a computer to use. But...I am considering whether an iPad would work in this case too. She could still use the MBP to do work at home and I could use the iPad while the computer was in use. Or maybe the iPad would work for her school work...taking notes and such. My guess is that it won't quite meet our needs and I'll go for a Mac Mini but I am curious to hear more about peoples experience with it.
I think whether this will replace existing machines, complement them, or not really serve a purpose will depend on the person. I suspect when I get an iPad it won't be because I need it, but rather because it will be something fun, and perhaps by then there will be some really interesting productivity thats suit my needs. For some people, such as my parents, I can see this being something they would like. I still thinks its really cool and have been excited to see it launch over the past few months. But for now I would say that unless you can see a need for a smashingly cool netbook in your life, then the only reason to get it right now is just because its really cool.
Edit: I would also add that some aspects, such as eBooks, may be reason enough for a lot of people to get it, and I am sure there are other legitimate reasons too...it just depends on the person. For me its not eBooks or browsing or streaming videos or magazines or things that like that have me interested in the iPad. It is things that may be possible with more apps and OS upgrades that will be the deal breaker for me.