I had an Audiovox XM unit w/ a cup-holder mount and it was a pain in the rear.

Recently switched to the Xact VISOR (which sells for $50). It fits neatly in the little slot just below the factory unit. During the day, it's tough to see the display through the plastic, but it looks sweet at night and in the afternoons. I put the antenna on the middle speaker and unless you look hard, you'd never know it was installed. With the agressive angle on the winshield, it works great.