Over many years, I have stood 20 or 30 times. Years ago, for decades, standing room cost pretty much the same as the cheap seats, so as long as you bought in advance, you didn't have to stand.
What I discovered somewhere along the way:
-- As mentioned, it is great knowing that you paid 1/4 of what the people 2 feet from you (whose necks you are breathing down) have paid.
-- I am fast...I have ended up seating during Act 2 for a good number of those performances, so that is a plus.
-- The better the show, the less you mind standing...so the first time, try to pick something that you are pretty sure you are going to like, based on familiarity with the score (if a musical) or word of mouth from people you trust, etc.
-- Try to pick a shorter show the first time...if you can't deal with that, you are in trouble.
-- Personally, I think it is harder to stand for a non-musical. In a musical, even if you are tired, the next musical number will frequently make you forget you are tired. With a play, I personally think that standing makes it harder to enjoy...it is easy to get distracted if your legs start to ache. (To this day, I am a huge walker; but get me to stand in one place for 10 minutes and I start to ache).
Years ago, I had a close friend who attended the same performance of Nicholas Nickleby, but stood...for eight hours; by the end of the performance, he was exhausted, but he loved the show.