I do not judge employee worth by whether or not I can see them working/goofing off. I judge them by whether or not they produce the goods on time and to spec.
That said I have a remote worker and it is a constant battle to see whether or not I am getting what I need out of him. He is poor at communications and so the rest of the team feel as if they do not know what he is doing at any time (so they assume, in the absence of any other data, that he is not pulling his weight).
I am sure that he works long hours and does indeed try to produce the goods but that doesn't take away the perception (if he cannot be contacted at 1000 people assume he is still in bed (and he might be for all I know)). If he comes into the office and spends time talking to people they see it as interference (with a dollop of "why is he sat here chatting to me instead of getting on with the work we are trying to complete").
I think it takes a very disciplined person to work remotely and those disciplines are: working at home as if you were going to the office; be fanatical about communicating with your colleagues; make sure everyone knows what your routine is (if it isn't the same as theirs) so that they understand why you cannot be contacted at 1000 in the morning.