I only have one at the moment, but I guess I'll probably use it for training in.
It was from last year's Race for Life, but I haven't worn it to date. I think I'm subconsciously trying to preserve it, but now I have a new one for this year's race, so I guess that's silly really.
I should start wearing them. I do have one rule though - if the t-shirt arrives before the race it doesn't get worn for the first time until race day! (Discounting actually trying it on, of course).