I used to do it much more often than I do now. I don't know what it is but reading and writing online, forums, etc. really seems to dull my capacity for creative writing - I think my analytical and creative thought processes just don't like each other much so when one comes out to play the other takes his toys and goes home.
When I get a spark, though, I can write for hours without coming up for air. And it is wonderful. Finished my novel for NaNoWriMo with about 10 days left in the month because I just got into it and flew. ...but I still haven't touched it since then for editing. x.x;