I have all three and found each one has its place. Admittedly my MBP usage has decreased dramatically.
I found that the iPad's really powerful and useful when you want to some light work, like surfing the net, reading emails, or even reading a book and you don't want to be chained to a desk. While the laptop can easily do the first two of those tasks rather well. I found the iPad to be better given its form factor.
What the laptop has over the iPad is content creation. Creating websites, editing images, writing documents, producing spreadsheets. Sure you can do all of those on an iPad but its an exercise in frustration. I found the better tool is the a MBP. Plus the MBP has more storage, and a file system to organize your documents.
so it all comes down to your willingness to spend 500 bucks for a device that makes listening to music, reading, playing games, surfing on the better then a laptop?