Does ping matter much under 30ms?

I play Quake-Live a lot, and hear people claim that they miss or are at a disadvantage because their ping may be 30ms while the person they are playing with will have 20ms. Sometimes it seems even more ridiculous like 20ms vs 15ms.

If a ping is that low, does goes lower have any noticeable effect or advantage?

Not to get all "get off my lawn, whippersnappers!" or anything, but as someone who has been playing Quake off and on for about 2 decades now, I can tell you that there's not that much difference between 30 and 15 msec ping. I used to play on dial-up versus people on broadband connections (where a "good" ping for me was 200-300 msec, and a "good" ping for them would be 50-100) and hold my own.

At 15-30 msec, I have a hard time believing anyone can really tell the difference. Human reaction time is not really anywhere close to these numbers, so I'd be willing to bet that if you hid that number and didn't display it, that a person couldn't tell the difference between them having the 20 msec ping and the opponent having the 20 msec ping.

There's many different aspects of networking and multiplayer gaming that can effect game performance, and ping is only one of them. Packet loss, client prediction, server performance, etc can all play a factor. It's possible there's some legitimate network issue causing people to play below their skill level, but "funky, unreliable networks" and "funky, unreliable game code" are both pretty universal, so everyone tends to be on equal footing most of the time, or at least on average.

That said, I've played with myriad people who always found a way to complain about something when they were losing, so even if you took ping out of the equation, there'd just be something else (ie, they have a better mouse or graphics card or more money/time to play, or the game makers/server operators have it out for me, etc) to complain about.