What exactly are they and how do they work?
In simply words masternode is a server on a decentralized network. It is utilized to complete unique functions in ways ordinary nodes can’t.
It's misconception that masternodes are an extension of Proof-of-Stake (PoS) coins (i.e. cryptos that aren’t mined, but are staked), but that’s not true.
There are Proof-of-Work (PoW) projects that make use of masternodes, so these kinds of nodes aren’t exclusive to PoS or PoW.
Do you think they're good or bad?
What is meaning of this? Do you mean in terms of owning a masternode and make some earnings?
Answer is not straight forward as it depends on which coin you select and how its gonna behave in long run..