This 2-cup Airtight Guac Saver works exactly as stated in the item description. It's perfect for a 3 avocado batch. The keys to success are making sure 1) the guac contains no air pockets, 2) the top of the guac is slightly convex in the middle (like a hill), and 3) the lid is pressed firmly down on the top of the guac. If you do these things, your guac will taste fresh and look bright green for days. I did notice a very thin (1-2mm) ring around the edge where the guac was a slightly darker green, but it still tasted perfectly fine. This ultra thin ring is no big deal and you likely won't even notice, unless you're specifically looking for it.
After doing daily visual checks and taste tests of our Guac Saver guac, I kind of forgot about it for a few days. When I found the 1/4 full Guac Saver a week later, the guac inside was STILL GREEN! I'm not kidding. It even tasted fine. Of note, my recipe doesn't include tomatoes.
Do yourself (or someone you like) a huge favor and buy the 2-cup Guac Saver. You'll save money, time, and will be able to enjoy green guac for days. Also, the 2-cup version (vs the 4-cup) is the way to go. I initially purchased the 4-cup version, but the diameter was much larger than I expected. The 2-cup version is perfect for 1-2 people and the small diameter ensures smaller amounts of guac and stay fresh.