It really really depends on your line of work.
I personally own an iMac, MBP, iPad, and a workstation (these are in my home).
I am provided a cluster (PowerWulf Supercomputer) and a workstation for work.
Now, on a daily basis I take advantage of every single piece of equipment I listed.
Typical days worth of computing:
-Go to work and bring my MBP (holds all my personal files).
-At work, use workstation to setup jobs and pass them along to compute cluster. When done, pull data of cluster and analyze on workstation.
-Read technical journals on iPad, keep references, contacts, emails etc on iPad.
-Before going home, pass on any after-hours work to MBP (or to bring along with me somewhere else).
-When at home, load files onto iMac. Setup cases and pass them along to home compute workstation (just a linux dual i7 24gb ram box with no monitor).
-Browse facebook and goof around on the net on iMac/MBP
-Play games on iPad, read books, watch movies on iPad
Rinse and Repeat
Now many people think I am overdoing it. I technically have 5 computers to myself, but as you can see, I put every single one to use. For example I am currently on my MBP, while my workstation is running computations, which will be ready for tomorrow mornings meeting.
Common questions:
-Why don't I eliminate iMac and just use MBP at home to pass jobs? Have you ever had 20+ terminals + visualization software + folders + papers open on a laptop at once? Not only can the laptop not handle it, theres not enough screen space.
-Why don't you use workstation for case running + visualization + home computer? The cases run over night and have 100% processor/memory utilization... Can't really use the computer while they're running.
-Why not eliminate that MBP? Need to be portable for work, yet have enough compute power to open data for post processing... i7 MBP fills the job
-Why not eliminate workstation at home and pass jobs to cluster at work through VPN? Data sets are over 10gb typically. Comcast already hates me enough
I'm actually considering picking up another computer just to fill my needs.
So back to the point... It really really depends on what you do with them. My case does not apply to a college student or soccer mom, but I know people who have blade racks in their house (which would be over kill for me)
Onto your questions:
I'll tell you from experience, if you do any decent computing... using a laptop as a desktop replacement will suck. I tried one of those dock things + my MBP in place of an iMac, and ended up just getting an iMac.
I prefer to read on the iPad (GoodReader is great if you read huge amounts of PDFs). I have a difficult time reading on my MBP.
I don't like iPhones. I use android because I can use ssh with xterm on it (in case I'm stuck without my laptop and need to connect to a workstation/cluster)
The combo will completely be up to your needs. If you do light computing, an Air + iPhone should be fine.
Efficiency in my eyes is having the correct tool for every job, I am not a swiss army knife kind of guy. If i need a steak knife, i get a steak knife.
