I buy the jacket every year but refuse to wear any of them until I'm old, at which point they'll be more about nostalgia than anything else.
Personally, I think it looks tacky to wear the jacket on the race weekend or soon afterward. It's like a 16-year-old wearing the concert tee shirt at school the next day - a move that just screams "hey look what I did, please ask me about it so that I get some attention and can talk about myself". It's the same thing as a dorky bumper sticker.
I have respect for anyone who completes months of training and runs a marathon. But the excessive self-congratulation that happens every year in Boston, personified by the ubiquitous jackets, is pretty ridiculous. The worst part, IMO, is at the airport the day after with every fifth person wearing the jacket and their medal while toting a backpack that has the race poster in it. It's fine to have a momento or two, but come on, we're in overkill territory here.
So to answer your question, buy the jacket if you want one. But please keep that thing in your closest until next year or later when you want to reflect back on your memories of the event.