For CRJ and Embraer flights on United and USAir, I carry a backpack with electronics and valuables, and an 18" vertical rollerboard for everything else. (Bought 2 at Target years ago, no longer available, but similar to the Riccardo 17")

The roller fits in the overhead if not overstuffed, the backpack under the seat in front.

The exception is 1st class in a CRJ-700 and some Embraer models (if memory serves), where the overhead space on one side is so small neither of my bags will fit. If I'm seated in 1st, I'll voluntarily gate check unless I have a tight connection in which I will try to board first.

I have never been asked to check the 18". In fact, more than once I have approached the gate for a tag and been told it's not necessary.

It helps to sit next to someone in the waiting area who is carrying a standard 22" roller. As the gate agent walks around issuing tags, yours will look tiny.

Many of these small rollerboards are geared toward carrying your PC and such, but I prefer one that is suited to clothing in case I am ever forced to check it (which, as I say, has not happened.)

NB: I am not a regular CRJ flyer -- this is based on roughly 18 CRJ flights in last 12 months.