Has it made a difference in keeping your nic's color and freshness longer? Any downsides or things I should know?
If your nicotine is stored in bottles with little or no headspace, then there is no real need for the wine preservation spray. In my opinion, leaving no room for any air is the best strategy. However, if you have any air space inside your bottles then the wine preservation spray will displace any oxygen, provided you use something like polycone caps that is going to hold in the gas.
Oxygen is not good for nicotine. So it makes a difference if your nicotine solution is at room temperature or refrigeration temperature. I don't know what difference it would make in the freezer over time but in my opinion it is a worthwhile investment in case it does make a difference.
One thing I learned the hard way regarding that wine preservation spray is that you have to be careful with it. If you spray too hard or too close to your nicotine solution, you can splash nicotine droplets all over, which can be a hazard.
Another note, I wrap my caps in silicone tape just for an extra seal.