I just came accross an article at The Economist entitled The Disposable Academic: Why doing a PhD is a waste of time. This has prompted me finally to write this post, which I've intended to write for a long time (like so many other posts on my too-quiet blog).
There is one, and only one, reason why you should go to graduate school in Physics or Astronomy. (This is probably true for any other field as well, but I'm going to stick to the field where I actually know what I'm talking about.) That one reason is: because you want to be a graduate student in physics for five or six years. That's it.
It is true that if you want to teach physics at the University level, or that if you want to have a career in physics research where you're leading and doing you're own research, you need to get a PhD. This isn't 100% true; you can certainly teach at the community college level with a masters' degree, and you can get a job working with a physics research group (although those are quite rare). However, for the most part, it's true. This leads many people to conclude that, because what they really want to do is spend their life as a professor at a University, they need to go to graduate school.
However, going to graduate school because that's what you want to do is similar to buying a lottery ticket because you want to be a millionaire. Yes, buying a lottery ticket is a prerequisite for winning the lottery, just as getting a PhD in physics is a prerequisite for being a physics professor. However, the fact that you've met that prerequisite is very far from assurance that you'll be able to do either. Thankfully, the chances of getting a physics professor job aren't quite as bad as the chances of winning the lottery. However, in both cases, they're bad investments.
There is a tremendous opportunity cost associated with being a physics graduate student. It's not as bad as being a humanities graduate student. For the most part, if you can get into a physics graduate school, your tuiton will be paid, and you will receive a stipend of something like $20,000/year. You may be able to make this as a research assistant— a good deal, because you're essentially being paid to do your PhD research. Or, you may have to teach some classes... which I also personally view as a good deal, but that's because I like to teach. (And, the teaching you do as a PhD student is lower stress and less time consuming than what a professor at a small liberal-arts college does.) However, there is still the opportunity cost. With your skills and abilities, you would be able to make a lot more money doing something else.
If you think you want to pursue a profession in academic physics, but you are going to view the years you spend working on your PhD as a sacrifice, then it's not worth doing it. The probability of getting that academic research job is just not high enough, even if you go to one of the top schools out there. What's more, ironically, the experience you get doing something else may well serve you better for any other job you might get thereafter, and it will almost certainly look better on your resume than the PhD will.
On the other hand, the life of the physics graduate student isn't necessarily a bad one. Yes, you will spend several years of your young life making a whole lot less money than you could otherwise. Yes, you will live the "graduate student lifestyle", meaning that you're still more or less pond scum in the hierarchy of your institution, and that you're still in training, still living the life of an apprentice. However, you do get to spend five or six years studying very interesting stuff, and performing original research. It can be a very cool thing to do. Yes, no matter who you are, you will go through moments of self-doubt where you wonder just what the hell you're doing, and you may go through periods of despair. But, overall, it can be a very fulfulling way to spend several years. That is, if you go into it recognizing that you're doing it for the sake of doing it, not as an investment in a future career that you'll have any assurance of achieving.
And, of course, to enjoy the graduate student lifestyle, you have to keep some perspective on life. If a professorial job were guaranteed, then perhaps one could stomach the idea of living several years with your life on hold, being underpaid and undervalued for working too hard. But, since that professorial job is far from guaranteed, you can't sacrifice your whole life to be a graduate student. Some will consider this heresy, will believe that graduate students are supposed to work really really hard because "your education is an investment in your future". But, again, a PhD program is today a terrible investment. Yes, you should probably expect to work up to 50 hours a week... not because you're overworking, but rather because you're inspired by your subject. But you should not, under any circumstance, join one of "those" labs where the professor expects you to work 10 hours a day, 7 days a week. You need to have a life. Work hard, but keep perspective. Recognize that you need to value your life right then.
What's more, you'll need to recognize that the culture of the PhD program is a bit dysfunctional. You almost certainly will feel cultural pressure to want to achieve the highly valued research professor position after graduate school, especially if you go to a top tier graduate school. You will feel this pressure from peers, and from your institution. (They partially judge the "success" of their graduate program based on the "placement" of their graduates.) Take it all with a grain of salt. It's your life. You are decidedly not a failure if you don't get one of the vaunted research positions, and indeed there's nothing shameful about deciding that you don't want one. Try to get one if you want one, and it's inevitable that you'll be disappointed if you don't, but don't feel ashamed, don't feel like a failure, and don't feel like you're letting anybody down if you don't get one. After all, most of us, if we're honest, will admit that we're overproducing PhDs in all fields, including physics, for the number of jobs out there that Physics PhDs are "supposed" to want.