Also, having a laptop spend hours directly on your lap may not be good for your reproductive organs. Not sure on this one, though.
The configuration I end up using the most is with the laptop sitting flat on the floor, one foot resting planted next to the laptop, chin resting on that leg's knee. That side's arm snakes under that knee toward the keys. The other leg is tucked in like cross-legged, other arm's elbow barely touching that leg's calf, straight to the keys. This setup puts my chest against the thigh of the upright leg with the chin on it. It's a super relaxed configuration, and I'll just alternate which knee is up vs. tucked in periodically.
This setup works quite well with a wireless keyboard that has a built-in trackpad. The keyboard can move between the table, the floor, your lap, etc.
It's actually more annoying than just having the laptop on the floor and leaning my chest forward against a bent up knee.
But it is a nice option for variety sake, and in my setup it's also the orientation required to enjoy the windowed view outside. In terms of pure ergonomics and computing comfort though, it's not as good as just the floor.
There's also the whole multihead display thing that the low desk provides, it's always a compromise.