I have a well-isolated vocal booth, and I have a home-made isolation baffle similar to this. My judgement follows other assertions made here: If you have a good room, the sound will be (at best) unchanged. In a noisy or less-than-ideal environment (like my band's rehearsal space full of other bands in rooms adjoining ours) it can make a huge difference. I've also used it to isolate hi-hats from OH mics for drummers who can't stop bashing them.
That said, you can make them yourself rather easily, and you can find pre-built examples for under $100.