Such passwords will be as random as your fingers can be. Humans banging on keyboards tend not to be very random; for instance, they will often alternate between left-side keys and right-side keys. As a rule of thumb, such a passwords would probably be reasonably secure if you make it long enough, about twice as long as a random password generated with a good randomness generator.
The main problem with a password crafted with such a "manual process" is that it is very hard to quantify its randomness. Especially if you generate several such passwords: I would expect successive "keyboard-random" passwords from the same human user to exhibit similarities. Thus, you have to overdo it in order to regain some guarantees, which means longer passwords (and nobody likes to type long passwords).
I recommend using a strong random generator. Dice will do; coins too (but a d20 will give you more randomness per throw than a coin). If you happen to have a computer available at that time, well, it can be used too.