I've been a cub leader for about six years, and an assistant cub leader for 4 years before that. It's wonderful, and I absolutely love it, way beyond my expectations. I was a Venture Scout as a teenager, and it feels great to be giving something back to Scouts.
The one thing I would say is that to do it properly takes a huge amount of time. There's a lot of admin and training to do, on top of planning and running the weekly meetings and planning any extra activities/camps. I was very surprised by the increase in the work load moving from assistant to main leader.
I'm lucky, I have a fantastic team of leaders, there are 5 of us running a pack of 32 cubs, and I wouldn't really want to do it with fewer leaders than that due to the size of our pack. We have a planning meeting each term, and split the planning of meetings between us, which helps. I do plan most camps and extra activities myself though, as well as doing/checking risk assessments, sorting badges, ensuring we have correct ratio, communication with parents etc on a weekly basis.
Cons - drain on your time, responsibilities, training. Camps are wonderful, but utterly, utterly exhausting.
Pros - the cubs! They're just fab, we have a laugh, and adults and cubs enjoy themselves immensely. It's wonderful when you see the cubs achieve things they didn't think they could, and you know you had a hand in that, and are giving them memories that will last a lifetime.
The friendship and camaraderie between the leaders is great as well, one of my leaders calls us his second family.
If you are thinking of doing this, I would check that you will have sufficient support (how many leaders will there be, will you be the leader in charge etc?). And I don't think I would enjoy it nearly so much without the use of OSM (Online Scout Manager) which helps with admin, badge records, activity planning etc. So perhaps check if the group uses this.