Depends what you mean by "feeling like shit". If you can feel an illness coming on or you haven't recovered properly from the previous workout then don't go - come back again another day and hit it hard. On the other hand if you're just a bit tired then get up off your ass and train - no excuse for laziness. Get some decent music on the go and get a good meal in before training. Some of my best workouts have been done when I've been "feeling like shit" an hour earlier. Other days I've been feeling great and had bad workouts - can never understand it.
Been off since Monday with a cold. I was going to go to the gym this afternoon because I feel better. However, I am still coughing. The last thing I want to do is have this shitting cold last any longer....I let my wife talk me into not going to the gym today. Looks like I'll be giving myself a full week to recover at this point. Bummer!
I agree that some of my best workouts have been when I really didn't feel like going to the gym but I went anyway. Once one starts pumping the weights the mood of the moment takes over, I believe. Not sure about the meal before training. Doesn't eating a big meal usually make one feel rather lethargic? I have also had days when I was in a great mood, really looking forward to the gym and then had a disappointing workout. It's just crazy!