I just got my GeekDesk after a long and varied string of various sit/stand combinations. I opted for the small, frame only, and bought my own "butchers block" solid mape top - 48" x 25" x 1.5".
This coupled with an ergotron LX monitor mount and a humanscale keyboard tray is beyond fantastic. I can adjust everything, and I love it.
This is very similar to my setup[1]. I picked up a geekdesk frame on craigslist and threw an Ikea countertop[2] on it. I even have a humanscale keyboard tray that I haven't gotten around to installing.
All in all, it probably set me back about $1000 (including the keyboard-tray and monitor-mount). And it was worth every penny; I'm much more comfortable after long hacking sessions.
This coupled with an ergotron LX monitor mount and a humanscale keyboard tray is beyond fantastic. I can adjust everything, and I love it.