It depends a lot on the laptop. This might sound daft, but from my experience of using various makes and models over the years, if the manufacturer calls it a Notebook or called it this at the time it was purchased (e.g. Dell's earlier models), it will likely either run uncomfortably hot or have a vent in a wrong place to comfortably use on a lap.
Some older laptops such as Pentium 4 based use to run hot. Those with fan vents on the bottom near the right or left also tend to run hot on a lap due to the vent being blocked. Some Dell models up until recently had this issue. For these, a cooling pad would be required, especially with anything CPU intensive.
Pretty much all Netbooks I've come across run comfortably on a lap, mostly due to their low power consumption, making them cool running.
Most recent new laptops have the input vent in the middle at the bottom or back, which means that these are not blocked on a lap and thus should run just as well as on a table. This is the way our Toshiba Pro A120 is and I've never had any issue with it getting uncomfortably hot.
To answer the question, I do mainly use mine on my lap, as I generally use it while out & about. For example, as a passenger in the car or on the bus, there's simply no table to place it on. :disagree: