I remember someone telling me that snails reproduce like rabbits.
What is the upside and downside to having snails in the pond.

The reasons I have snails in my pond are:
-they are interesting to watch
-they eat lots of algae

I don't have too many snails yet... so the jury is out as to whether they will be annoying or not! (What will I do with extra snails? Likely give them to my mother-in-law, for her pond... her snails get eaten by birds, so she always wants more!)

I list the pros and cons of snails on my snail page at http://www.fishpondinfo.com/snail.htm#pro

Some species of snails reproduce quickly - pond snail, ramshorn snails, trumpet snails, etc.
Some species reproduce slowly like the Japanese trapdoor snails which give birth to live young. They're the best snails for ponds. They don't eat plants or cause many problems. Raccoons keep eating mine.
Apple snails reproduce slowly but grow large, eat plants, and have to come inside if it goes below 55 degrees F.

I suggest adding some trapdoor snails and black ramshorn snails. No matter what you do, if you add any live plants, even if you try to treat them for snails, you'll eventually end up with pond snails. They're just part of the pond and always find a way. I used to treat the plants when my pond was new with potassium permanganate (a lot of work to soak them, rinse them, etc. plus it tended to kill the plants) but the snails ended up in the pond anyway.

Raccoons love to eat snails. Some fish will also eat small snails. My 1800 gallon pond with orfe, koi, and goldfish has very few snails except in the waterfall area. My other ponds without fish have more snails and sometimes do damage plants.