I currently own a laptop, and that's all i have owned for the last 4 years.
However, a few months ago i understood the big advantage of having a desktop: updates.
My laptop was quite good when i bought it and, as nontroppo described, it was good enough to do everything i needed at the time. Unfortunatelly, with time, it became too underpowered. So, i had to add it more memory. I payed about 2 times more than i would have payed for the same ram for one desktop, and the desktop ram would be faster, and wouldn't force me to remove the ram i had at the time (which the laptop did).
Thus, as i'm nor rich nor stupid, and since i already have the external keyboard, mouse and screen, i decided that in the start of the next year, i'm going to buy myself a decent desktop for half the price of the equivalent laptop, which will last me twice the time that my new laptop would have lasted
Since i've mentioned the external keyboard, there's another important factor: my previous laptop's keyboard got ruined, some keys weren't working. In a desktop, that wouldn't have been a problem. In a laptop, that also isn't a big problem, it only costs 5 times more
(oh, and the keyboard lasted 2times less than my current external keyboard already lasted!)
So.. Make the maths