It's always an approximation of what I have in mind, but, more often than before as I learn, I do get closer to my goals.
Still, it's funny how sometimes some things will work out okay in your head, and you really think that you hear that stuff as clear as if you were actually listening to it on a cd or something - but then you record it and it just doesn't work, not by a long shot. That's one of the weirdest things.
Then there's the obvious compromises - maybe you hear John Bonham on drums or you imagine that awesome piano riff but you have to resort to MIDI sequencing and you don't have access to anyone who could really nail those parts.