I tried it with and without recently, I found that for languages I'm familiar with having it on or off made no difference however having flag errors on off did.
For languages I don't use much I found having it on more helpful, could I work without it of course, I grew up with computers that didn't have syntax highlighting but I'd miss it.
One side result of the experiment was that I turned down the number of colours I used, not everything needs to be a different colour.
The only synhi I've ever not actively hated was the elvis defaults on a slackware install ten years ago, which basically bolded/italiced/faded text but didn't change the colour.
For languages I don't use much I found having it on more helpful, could I work without it of course, I grew up with computers that didn't have syntax highlighting but I'd miss it.
One side result of the experiment was that I turned down the number of colours I used, not everything needs to be a different colour.