Like most Pink Floyd material, it is depressing. But this particular song struck a chord with me and I found it inspirational instead. When you're young and stupid, you have this feeling that life is so long as to be virtually endless, so you don't a damn about wasting days, months, even years, because whatever, there's always tomorrow to get your shit together. You kinda get used to your parents telling you when things need to get done, and kinda get used to ignoring them if you want, or if it doesn't seem like there will be any real consequences.
Then one day you realize that you've been totally fucking up. Your life is happening, it's passing you by, while you sit on your ass and watch TV or listen to tunes. No one told you when to run; you missed the starting gun. No, fuck that. Your parents, your teachers, every adult you've ever met has been telling you for years to get your shit together and make something of your life; you just got so used to ignoring them that it never occurred to you that they might be right.
It was years too late (but fewer than 10) when I finally realized what this song meant, and I'd been listening to it since I was in junior high. Ever since then, I've lived my life to make the most of it. Yeah, you're gonna die anyway, but there's nothing you can do about that. What you can do is live your life to the fullest in the meantime.