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I think it really comes down to your needs. I have i7 iMac at home and an i7 MBP I use for work and on the road. I live in a 1 bedroom suite and when I am at home it feels like overkill to have these 2 machines in the same small space. I have considered going with just a MBP and monitor. But the quad core iMac is so powerful it is really hard to part with.

I have considered using other combinations of machines but this set up works the best for me and is what I am used to.
 
I have a 15" 2008 model MacBook Pro and a 2010 27" iMac. They complete each other great. I have my MBP for at school and iMac for at home. It's great.
 
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