I prefer filling them manually myself.
I use a 16oz water bottle. Pour the distilled water into the 16oz bottle and fill one at a time.
Buggies Unlimited sells a single point watering system that is supposed to work good.
I think the water systems are great, they save you from splashing acid and every time you open the battery caps you let in dirt. If you don't trust them you can pull the caps off once in a while to make sure all levels are there. .It only takes like 2 minutes to fill all the batteries. If you have Trojan batteries they make they own, or use Pro-fill.