Well I finally purchased my first good quality set of ear plugs. They're the Etymotic Research Musician's Earplugs with both the 15 db and 25 db attenuators. I was expecting an improvement over the regular $10 plugs, but I was blown away by how good they sounded. Very smooth frequency response, nice high end. It starts to roll off quite a bit above 8 or 9 k, but that's hardly a deal breaker.
I used them all during the festival of lights in PEI last weekend, and it made the shows much more enjoyable, except for the Hip, when I forgot them at home. They're the first hearing protection I've used that I feel like I could mix with. Would take some getting used for sure, but once you get used to the new balances and the slight loss of HF, I don't think there would be any problem. Generally I don't do a lot of gigs where hearing protection is really required, but it happens.
I think everyone who enjoys going to concerts, not just musicians and engineers, should have a pair of these. Best $300 I ever spent. Worth every penny.