Best time of year to visit Berlin?
 

I know there generally is no bad time to visit a city as such, but i would really prefer to go when it at least wasn't raining. Is it better to visit Berlin in the Spring (April/May time) or the summer (June-September)?
Also, is it possible to see Berlin on a long weekend trip (3-4 days) or would it better on a week-long trip?

April-June are fine, July and August can get really hot.

In general, Berlin is in an arid region of Germany. It rains appr. only half as much as in Munich or Hamburg. So, statistically, your chances for dry weather are pretty good.

May or June would be my favorites. In June, there is hardly one weekend without a major festival going on - in case you like that.

Berlin is a typical weekend trip destination. But you can as easily spend a week without getting bored. Especially when you include the sights in neighboring Potsdam.

I would recommend any time between late April and mid September.
Days are fairly long during that time, there is a good chance of fair weather - and as Berlin offers many outdoor attractions apart from its museums that comes in handily.

The weather is somewhat unpredictable, was hot and sunny for like six weeks last year then rained a bunch, but who knows what this year will bring. Generally speaking summer is great, but perhaps May would be better.

Besides this I really wanted to recommend a weekend trip to you. I only say this because outside of the tourist sites, which I think can be done in a day and a half, you can really get a feel of the city on the weekends.

Head off to Mauerpark on Saturday or Sunday (though Sunday is better) to see the open-air Karaoke and on Sundays to see a GIGANTIC flea market. There will be anywhere from 4,000-10,000 people around, generally young having fun.

You could also buy a cheap grill, some meat (groceries are cheap in Berlin as is beer) and head to Görlitzer park or Treptower park and go grilling. It is easy to socialize and very chill. At night head to Kreuzberg or Friedrichshain for the night-life of Berlin. Tons of clubs, bars, and in between. Explore the night till 8 in the morning, many others will be. People are friendly and international and it is hard NOT to meet people, as long as you are open minded.

If you are interested in history and art, you could spend more than a week in the city .
I was there in 2009 in mid-July and was rather cool for the first few days, turned warm the last day ; mid- Sept 2010 was nice ( rained one day all day out of five).
Going again the first week of July.