I umpire for Bellaire LL in Houston and used to few a couple others in the district since I graduated about 9 years ago. I love it and if I could do that as a full time job I would in a heartbeat. It's up there with crawfish season and Astros season as one of the things I look forward to most in the spring.
What you can expect is wildly different depending on the quality of play in the league. For example Bellaire has a lot of history of high quality play, and their parents, coaches, and kids take the game very seriously. I appreciate that to be honest because they understand more of the game than just "something didn't go our way so it's the umpires fault".
With other leagues, there may be a lot more teaching involved from the umpires. It is not uncommon to be literally teaching and explaining some rules to coaches in the middle of the game if they come out to argue something.
Either way I can't recommend it enough. Are there occasionally jackass coaches or parents? Absolutely, but not nearly as often as people might lead you to believe. The kids are fun to be around, it's great to be involved and outside, and it's baseball. What more could you want.
Disclaimer: you will definitely have an urge to go and take batting practice off of the kids every time out though I promise. It's not advised to crush dingers off of 11 years olds though.