I was wondering about the usefulness of industry conferences and learning seminars. Is it worthwhile to go even if your boss won't reimburse you for your expenses or pay for your time?
It really depends on your goals. I find that if I skip one of the conferences that I regularly attend, I miss a lot of emerging technology. I never go to conferences that don't have a vendor show tied to them. I always get a lot more from looking at the stuff that people want to show off than I do from most of the presentations.
Your post sounds like you are a recent grad. For you, the presentations might be fresh and new and amazingly worthwhile. I remember when I felt that way and I got a lot out of them. After a decade or so the bloom goes off that particular rose and the show part of the conference becomes my focus.
If you are just starting out, I probably would recommend one of the big shows (like the ASME World Conference or the SPE Annual Technical Conference and Exposition if you are in Oil & Gas, maybe Offshore Technical Conference if you work offshore) even if you have to take vacation and pay the T&E yourself. Actively look for people to BS with (networking is prime benefit of these things), give away a lot of business cards, spend time in the show. I always find that thinking about a particular problem in my job and trying to find a gadget in the show to solve that problem helps give the show a purpose in my mind and makes it more fun.