Eh... it depends. I used my tibook in my bed or on my chair (laying in bed and using the chair to set it on so I can not worry about fall asleep on it

) every night for 4 years, and it was fine (never broke down, no problems AOK.. and those things got TOASTY) and now I use my current SR refurb macbook pro on the chair too and this thing is COOOOL even dvd paying and doing solidworks, it's GREAT.
Personally I am in college, but in general, I use the portability even more at home, I love sitting in different positions and on the couch ot bring it to the kitchen to look up recipes on, or to listen to music, hook it up to the tv to show a presentation or a downloaded snippet or random things for friends. It's nice to be able to break up the monotony, i find myself getting tired and achey in one place constantly, it really makes work better, and I find myself moving around more and feeling better with the laptop portability.
I recommend it. The 15inch is a nice size. I have an external monitor 22 inches for big work, like rendering or making my final presentations, but all in all most of my work is on the 15inch screen.
Personally, the laptop is a great thing. IF you can ever afford a screen (you can get a decent 22inch like mine for under 200 online... that's relative christmas money

) The setup with the laptop in clamshell mode doesn't take up any more space then the imac really. If you want super quality screen, you can probably get a good 20 -22 inch for 300... and it's something you could use for the next few years once you go to college and have a laptop anyway. Save up a few bucks if you ever feel yourself craving extra space, but you probably won't need it really.
My bf has HORRBILE vision (recently had to get a cornea TRANSPLANT because of the kerataconas (sp?) and still has to get th eother eye done... and he's had a 15 inch laptop (now a mbp) for years, and he uses it as his primary and only computer screen... and he's fine) Spaces will help a lot too for any intense applications.
Goodluck with your decision. Either way you'll be happy, but portability is NICE.