[Serious] Generally speaking, is philosophy a worthwhile subject of study?

[Belacqua]

Philosophy is wonderful to discuss with friends over drinks (Symposium, anyone?) but is rather useless as a source of knowledge.

That depends on how you define "knowledge." 

It's wonderful as a source for certain kinds of knowledge.

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Then there's also the issue, I don't know whether this is valid or not, that it seems philosophers aren't necessarily always even making an argument, that could be reduced to such a logical form, more just making observations and asking questions, which may eventually feed into an argument, but don't necessarily do so in their own right, thus making it even harder to discern what is relevant when trying to reduce a large body of text to a simple logical argument.

This is certainly fair. There are different kinds of philosophical texts, with different ambitions. 

This seems to be the case right from the start. Plato's conversations bring up various ideas and possible answers which are discussed and then put aside. His characters tell myths to illustrate what an answer would be like, without actually giving an answer. 

This frustrates people who expect every text to be like a science book, with the answers written out in simple declarative sentences and little blocks of text inset in the page to define the hard words. But, as with all good books, Plato demands that we use our brains to their very limit, and his texts' value lies largely in that they have provoked inconclusive conversation for a very long time. 

Aristotle, arguably, is more in line with what modern people want. He doesn't use myth or allegory, he tries to build up declarative statements into logical arguments to arrive at clear conclusions. So he's a model for some later types, and you can categorize people into Plato types and Aristotle types. Both can be fantastically difficult. And this is not made any easier by the fact that some people (Nietzsche and Adorno, among others) think that writing their ideas in simple, third-grade level sentences, would actually contradict their ideas, and argue tacitly against them. Oversimplifying is falsifying. If the "medium is the message," then a difficult medium is an important part of the meaning. 

Modern people tend to forget that there are other ways of constructing a text besides the way that a science book or a good newspaper article is written.

you are highly immersed in the subject as a whole... or at least within the particular branches of it that interest you.

Thank you, this is kind of you to say. 

I've been at it an embarrassingly long time now. And I did take the time to get an advanced degree in a related field, so that was a few years of swimming in the deep end. 

But I'm frequently reminded that I'm at a low level compared to many others. For example my own research referred to Jacob Boehme's writings quite a bit, and showed the link between that and later German Idealism, particularly Hegel. And Boehme's importance in England. But recently I've come across a vein of research showing Boehme's dominance in the other wonderful, but less read, philosophers like Schelling -- this is terra incognito for me, and I would love to live long enough to read it all. 

But if you are feeling a little imposter syndrome, or just that you're a in a bit over your head, that's a good thing. The opposite -- to believe you know more than you do -- would be far worse. (And there are plenty of people like that.) The fact that you don't believe you know The Truth is extremely important.