The office culture and opportunities varies A LOT by the boss (the Senator, the chief of staff, the LD). If the boss is good, people will stick around for decades; if not you'll see a lot of turnover. My office was not at all interested in staff professional development or education, I had to make my own effort there. I saw another woman endlessly criticized for needing to leave on time (not even early! on time!) to take care of their kids.
The pay is generally lower than what you'd get in the private sector. A lot of bosses will make appropriate pay decisions, but a lot know that an army of 24 year-olds will do the job for free, so you don't have a lot of leverage in pay negotiations. That's why so many people go to lobby after-- it's easy to say they're "greedy" but living in DC is expensive.
Office politics rule - the bosses' buddies get the jobs, promotions, raises, period. A lot of opportunities are never even posted and just handed to a favorite. Likewise, an "inner circle" springs up of these buddies, and they make the decisions. If you're not in the group, you're chopped liver. But, they're very defensive and are not interested in changing, so be prepared to play that game or get pushed out.
It's a bit like a customer service job - most constituents will be reasonable in their expectations of you and the member, others will not. Some will be very abusive because to them you personify "the government," even if you're trying to help them. Similarly, even as a staffer you have to be very careful about your public/social media conduct as the media/constituents are watching.
