Depends on the conference and what you expect to get out of it. There are some conferences--usually sponsored by colleges--that are basically writing workshops. A lot of hands on work, but no agents. Others have workshops and events involving agents. The one I go to every year is more of a networking one because of the area I'm in--though it does have workshops.
Favorable locations is any place you want to go to and can afford to go to. I went to ThrillerFest a few years back--not a big hike location-wise, but the cost was really high. Hotel and parking was expensive, and the conference charged for everything. Fantastic workshops, but not great in the networking department. My regional one--only fifteen minutes away for me--tends to have non-fiction workshops (I'm fiction), but I always get the opportunity to meet agents. They have an agent breakfast, where you can pick an agent and eat at a table with them. Lunch is a free for all where everyone eats together, so you may find yourself seated at a table with five agents. Plus the pitch sessions, as well, and an agent roundtable (I believe last year they had both non-fiction and fiction; I actually have never attended any of the workshops).
I run the agent pitch room at the conference. If you are pitching, your novel MUST BE DONE. Every year, we get one or two writers who pitch an idea or the first three chapters of an unfinished book. I can tell you it really annoys the agents. We even had someone who tried to pitch a short story. This is just wasting everyone's time.
Even if you're not pitching because your book's not finished, have an elevator speech prepped. If you're around agents at all, they'll probably ask you what you're working on. It's a great opportunity to practice, even if the agent doesn't rep your genre. Plus, you never know. He might refer you to another agent in the agency.
By the way, if cost is a factor in the conference, look into the possibility of volunteering. Some conferences will give you a discount on the conference fee, plus you may be put at a table to welcome people. Do ask questions though--not all conferences are volunteer friendly. One of the other ones I attended was a little snobbish in the volunteer department and made it extremely difficult to volunteer (and they were sending emails asking for help!) and didn't provide any discounts.