There have been a lot of great replies in this thread, many of which I've read, some which I just scanned, and one which I missed because the poster was on my ignore list. My league experiences have tended to be quite good, but I think that has had more to do with my expectations than the actual merits or shortcomings of any particular league. At the risk of not adding anything new here are my thoughts. Hopefully they with help the OP with his questions.
1. If money payouts are important to you then you will not be happy with any national league. I've played in several different league organizations and I can say with confidence that no player or team ever makes more money than they spend over a season or session. (This is taking into account membership fees, nightly fees, table fees, drinks, food, transportation, babysitting, and any other expenses that may be associated with league participation.) The outlook is even less profitable if you attend league tournaments, either local, regional or national. Leagues are about bragging rights and social interaction much more than they are about profit for the players.
2. If you find it hard to ignore or deal with drama and politics leagues are probably not for you. Any time human beings organize into groups for any purpose drama and political issues will follow. It's just human nature. There are always going to be those that argue over things that they would be better off seeing as trivial, and those that gripe about how things are done without ever offering or being willing to work for something better. You need to be able to look past those people if you are going to be comfortable with leagues. (I'm not referring to politics as issues surrounding government or political parties. Rather I am using the term in a broader sense as defined by the Oxford Dictionary as having to do with, "The debate or conflict among individuals or parties having or hoping to achieve power.")
3. If you are considering leagues as a source of instruction, be very careful in choosing the team you play on. If your team has some better shooters than you, and they are willing to spend time outside of league nights helping you improve, it can work out well. Keep in mind, though, there is usually not a lot of time for instruction on league nights although you may be able to pick up a quick tip or two, or gain some knowledge just by watching. It's pretty much a sure thing as well that you won't get any useful instruction from opposing players, at least on league nights. Also, it's been my experience that the great majority of league players (including team captains) are not as knowledgeable nor as proficient as many of the players you will encounter in your home pool room. While we're on this point, keep in mind that you can be a source of instruction for other league players as well. I can't count the number of times I was able to teach other players, even ones better than I, something new.
4. If you are thinking of joining a league in order to get more table time in competition you could very well find that leagues are great for that. The level of competition is not always consistent in leagues, but they do provide the opportunity for head-to-head competition every match.
5. If developing friendships with other people of similar interests is one of your considerations, leagues can be ideal for that. Not everyone you meet in leagues will be someone you want to become friends with (or even be in the same room with) but I have developed some very nice relationships through league participation over the years. Forget using leagues to meet women, though. It can happen, but it's rare enough to exclude this as a reason for joining.
6. If you are unsure of whether leagues are for you or not I would suggest joining one. After all, how will you know if you don't try it. Keep your expectations rather low and your mind open and you might find that leagues can help you with a few of your objectives while providing some fun in the process. At worst it will be a few wasted evenings and a little money spent on nothing of value. No big deal, right?
I was reluctant to join my first league, having heard all of the complaints that are common on this forum, but I had a couple of friends that kept at me to join their (VNEA) team until I agreed to sub for them for one season. I liked it enough to commit to playing full time during the summer session, and once that was done I came on full time for the regular season. Over time I joined other teams and wound up playing leagues 3 nights a week for a few years. Then a marriage and family intervened and I spent 15+ years out of pool almost entirely. A few years ago I decided to pick it back up again, and leagues have been a part of that, albeit a much smaller part than before. I have played with VNEA, BCA, and several local leagues without national sanctioning. I have played in local, state, regional and national league tournaments as a part of my involvement, and I can say that although not everything has gone well all the time, overall it has been a very positive experience for me. It's just a matter of keeping a good attitude and being able to fade the BS.