I like that the fan keeps the beer lines and taps much cooler. The temps are more uniform throughout the freezer. I have a small commercial freezer which has a fairly large OEM fan. That's what prompted me to install one in my converted freezer.
The compressor will likely cycle more frequently as the circulating air will increase the heat gain. You can set a wider differential to counter this. I'm currently running a 10 degree differential. The off part of the cycle is about 60 minutes and on for about 20 minutes.
Insulating my 2 X 6 collar reduced the power consumption on my freezer by nearly 40%. Not a big deal as these units don't use much power at all. So, I highly recommend insulating your collar.
The fan noise is noticeable, but about the same as a refrigerator running. That type of noise does not disturb me, so I never give it much thought, but you can definitely hear the fan running. I'm sure you could find a super quiet 120 mm muffin fan without having to resort to speed control. There might be a way to muffle the fan noise.