In practical terms, these days, a credit union IS a small "savings and loan" bank -- the kind of bank that used to exist before bankers started making money on everything but writing loans. They aren't always going to offer higher interest and/or cheaper loans than the bank-banks, but they're almost always going to be more pleasant to deal with since they consider the depositors and borrowers their stockholders, not just customers.
There are minor legal differences (different insurance fund, for example), and you aren't necessarily eligible to open an account at a randomly-chosen credit union (depending on how they've defined the community they're serving), but they will rarely affect you as an account holder.
The main downside of credit unions is that, like other small local banks, they will only have a few branches, usually within a limited geographic area. However, I've been using a credit union 200 miles away (and across two state lines on that route, one if I take a large detour) for decades now, and I've found that between bank-by-mail, bank-by-internet, ATM machines, and the "branch exchange" program (which lets you use branches of participating credit unions as if they were branches of your own) I really haven't felt a need to get to the branch.
I did find that, due to network limitations of $50K/CU/day, drawing $200,000 worth of bank checks on a single day (when I purchased the house) required running around to four separate branch-exchange credit unions. But that's a weird situation where I was having trouble beating the actual numbers out of the real estate agents until a few days before the sale. And they may have relaxed those limitations since... though if I had to do it again, I'd consider taking a scenic drive to hit an actual branch of my own credit union.
If you have the opportunity to join a credit union, I recommend doing so. Even if you don't wind up using it for your "main" accounts, they're likely to be people you want to talk to when you're shopping for a loan.