Plus500 is a market maker CFD broker, but that does not make them good or bad.
I have not traded with them so this is my opinion only. Just because they are not a DMA broker, it does not automatically mean they are bad.
They are well regulated & they are also public in the UK, Plus500 is definitely legit.
Whether they are bad in terms of actual trading I don't know.
But from the spreads & all the charges listed on their website, they are definitely not cheap. There are lots of reputed CFD brokers out there that are also cheap.