i had season tix to the celtics, and split them (with a few friends) for the patriots and bruins on a couple of occasions.
to measure if its worth it really depends on the person and sport.
a few things i learned along the way-
i would never pay to go to an nfl game. not worth it, too expensive, takes up the whole day, too many timeouts, and too many druken assholes (but this could be because of new englanders in general). only time i ever went was with a large tailgate, which was more to party than for the game. or if a connection of mine invited me to sit 50yd line 10 rows up, then i'd go.
for the winter teams, even when the teams are good and it seems like a hot ticket, there are still plenty of duds. funny how everyone begs you to let you know when you have a game for sale, but can never make it when the nets are in town. however, you can break even on the shitty games if you post them on your teams 'ticketexchange' WAY in advance, as they will sell early for face value. shitty games are shitty, and week of you are lucky to sell for cost.
the thing that i always battled with was i'd rather sell the hot regular season games for a decent profit, as 1) i liked making the money and 2) hate going with a ton of commotion, traffic, and people who go once a year. so i'd find myself going to more midweek games, but those can always be had so cheap on the street, often i overpaid by paying face value.
The main positives are you have same seats, get to know people around you, teams tend to treat season tix holders pretty well. also, you can save quite a bit on the fact that your seats are approx. 20-30% less than individual game prices, and that is before ticketmaster tacks on another $15. My celtics tix cost me $36, and if you bought from tm they came out to 55.