I have hardly touched my "desktop" since getting my i9300. My lappie does everything I want and more. Besides, it runs at a higher resolution (1920x1200) than my desktop screen (1600x1200... it could do 1900, but only at 60Hz and it looked like ass).
So as a desktop replacement? Absolutely. 90% of the time it's plugged in on my desk at home, while my forlorn former desktop has been relegated to email and web machine. I even hooked up my laptop to my 5.1 surround system yesterday, and really dug the results
It totally qualifies as a desktop replacement, because there isn't anything I did on my desktop that I can't do better on my laptop.
Oh, with one exception. I was running a RAID array on my desktop to power the disk needs of my sound recording stuff for movie and video game soundtracks I compose. My laptop doesn't yet have the disk power I want, so I'm still reverting to my old desktop when I have to push mondo amounts of tracks. However, my old desktop doesn't have the CPU horsepower this one does, so I'm slowly "archiving" old tracks, and trying to keep working with 30 or fewer tracks at once on my new "laptop DAW".