About the climate in Munich.
I've lived here a good few years in Munich and in my experience the climate in Munich is like this:
Jan to March
Really, really cold, like down to -10 and sometimes -15. It's mostly dry, so a dump of snow lasts for weeks. If you come at this time, bring a hat, scarf and other warm stuff. It gets really cold!
April to June
April and May weather is variable, there can be showers, drizzly rain, cold weather, warm and it's generally a bit all over the place. Towards the end of May though, it starts to get nice and you'll see the street cafe culture starting up.
July to September
Very hot with thunderstorms. The summers in Munich are really hot and much to the annoyance of the beer garden owners frequently there are thunderstorms in the evenings. They usually pass quickly though and don't dampen spirits.
Update for Summer 2011
The summer so far has been really miserable, it's been rain rain rain with a few hot days. Hopefully August / September will improve. Make the most of any sunny days, they might be the last of summer.
October to December
Towards the end of the year not surprisingly it gets colder and rainier. If we're lucky it snows in time for the Christmas markets though the last two years have been annoyingly mild. There is nothing like standing with a mulled wine on crunchy snow looking at the snow flakes falling. Fingers crossed.
So when to come?
My advice to friends it to either come in the winter when it's nice and cold with snow, or come in the summer and enjoy the beer-gardens. Spring and Autumn can be a bit depressing.