How much do you plan to use it? How long to you expect it to last? Walgreen's has a rollaboard for $10 this weekend; if you're careful with it, it could last a while. If you can get an ultralightweight but relatively expensive polycarbonate roller at ebags.com or even Target, it costs more up front but lasts longer.
I have an underseat Travelon bag I got from QVC, but ebags (I think) has something similar. Smaller than your typical rollaboard, but it truly does fit under the seat in front of me. It's wedge-shaped with a handle on the bottom. Note that this is the only suitcase I bring onboard. My personal item is my purse.