A decade ago, the Black Diamond Neutrino was the first really small and light carabiner to gain popularity. It was dramatically lighter and smaller than anything else seen on Yosemite's big walls. I wouldn't do a speed ascent without them. Today, there are dozens of light biners available and the Neutrino has some stiff competition. In general, we prefer a biner like the Black Diamond Oz Carabiner or Wild Country Astro that is lighter and a little easier to handle. That said, many people still swear by the Neutrino. One cool thing is that you can buy them in colors that match the colors of your Black Diamond Camalots and Black Diamond Camalot C3s. That way, when you reach for a cam, you will know right away what you are going for. If you want the absolute smallest biner, check out the Metolius FS Mini or if you want one of the lightest carabiners, check out the CAMP Nano 22 Carabiner.
In February of 2016, Black Diamond released a recall of Oz carabiners - 210230 and 381052. If you think you might own any recalled carabiners, read this safety recall notice to determine your next move.