I'm using it and I like it!
The only drawback (for some) is the fact that when you dock with a milk cow, your current patrol ends and you will start a new patrol with fresh fuel and eels. If Sansal is ever able to find a workaround for this glitch whereby re-supply extends your current patrol, I think it will really come into it's own.
Also, you only have 2 cows (in Spanish ports) early on in the war, up to 1942 - the mid-Atlantic cows don't come into the game until about mid 1942 onwards.
Check it out.
Hawkers