I'm just a little bit into the second year of my PhD so I don't have the perspective of some others, but for what it's worth, I'm having a great time so far. Some of the other grad students in my group are decidedly not having a great time. But I think the difference between them and myself is that I wanted to do a PhD for its own sake, to keep learning about science. I'm not losing any money right now - in fact, I'm living comfortably and saving a little bit on top of it (although this varies between programs of course). And I believe that wherever I end up, I'll be able to make the best of it. If it's not academia, then so be it.
If you want to do it for the experience itself, then I think you will enjoy it and be happy. It's when you do it for the end goal that you get into trouble. Of course you need to have a long term plan, but there are people out there who think that you have to suffer through something now to get what you want later. And I think those are the people who are miserable their whole life.