In my opinion, and opinion is all you get in this thread, I would say yes and no. Some dacs do improve the sound (depends what you personally consider improvement), many dacs sound the same as each other, some don't do anything at all versus the source you connect to it (assuming that source has it's own dac that you were looking to improve).
I've been through about 9 or 10 different dacs over the past few years, be they stand alone units or inbuilt dacs in amps/ sources etc. Many do sound more or less the same with maybe a slightly different presentational emphasis. Some enhance treble, others bass. Some appear bring out more detail in the mids and so on. It really all becomes a matter of trying out different dacs and finding one that does what you like. What I can say with some confidence is all dacs do not sound the same, but it is up to only the listener to determine what sounds like an improvement to them.
Some will say differences are imagination only, I suggest they perhaps have not tried out a wide enough variety of dacs at a variety of prices. And no, more expensive ones will not necessarily sound better, not in my experience anyway.