I've been using them for over 20 years, first for the convenience of not having to check battery water levels, and second for the large reserve capacity. I use the group 31s and have 4 in my boat now. I'am on my second set on a 21' fishing boat that came with the standard lead acid type, which I replaced at 3 years. Mercury specs them for the big Verado engines, but I was using them for years before I got this boat. I need a lot of juice as I do mostly 3 day trips up the San Juan, and I can nurse three on a 36 volt troll motor to last for the complete trip. Down side is price, I use the Batteries Plus X2 31M, which has a free 4 year replacement and if you buy online and pick up at local store there's a 10% off coupon, which you'll need! I personally have had good luck with them and I have used Lifeline, Trojan and Sears Platinum Diehard brands. Ranger621