I'm about to graduate with a degree in electrical engineering, here's my flow:
iPhone 5: Well, duh, it's a phone. I use it for texting, checking email, social networking, perusing the interwebz, GPS, taking photos, etc.
iPad (3rd Gen): I mainly use it to take notes with in class. I have a really nice stylus that I use (I tried a handful before finding one that was actually useful). It's awesome with the app Notability, because I can change pen colors, line widths, highlight things, cut+paste, type, record audio, etc. People always ask me to send them my notes, since they're much more organized and colorful (which can help with studying). My notes also automatically backup to Dropbox in pdf format, so I can access them from any computer, and even my iPhone! Other than taking notes, I also have my textbooks on it, as well as the typical slew of social networking apps and games... but in all honesty, it's really only for academia and I'll probably sell it soon.
13in MacBook Air (2011): My main computing device that I consistently use for homework, surfing the web, writing reports, etc. Will probably be moving to a 13in Haswell rMBP here soon for the beefed up hardware and higher resolution (when scaled to 1680x1050, that is).
27in iMac (2011): Used to use it a lot, and sometimes I do if I really, really need to be productive (like with this take-home exam I'm procrastinating on...) or if I need the screen real estate. But I've since been seldom using it and it's really only serving as my media center (to stream movies/shows/etc. to my AppleTV). Will probably sell it soon and move to a Haswell Mac Mini + TBD.
Definitely like the form-factor of the iPad Mini though. Will definitely get one when it gets upgraded to the retina display (though may consider the 5th gen iPad if it's significantly lighter)
EDIT: And for those wondering, OSX has never hampered me in pursuing engineering. Any Windows-only apps I run via Windows 7 in VirtualBox (free) and any unix software (like Cadence), we typically VPN into our school's servers, so that's not a problem either. Office for Mac has served me swell and Mountain Lion has arguably bettered my work-flow (such as having multiple desktops/spaces)
EDIT EDIT: I also have a 6th gen silver iPod Nano that I wear on a brown leather wristband as a watch. It has music on it, but I mainly stream from Pandora on my iPhone. Nonetheless, it looks really snazzy and I always get a ton of inquiries/compliments about it. So in the end, you can never have enough Apple products!