I once heard a standup comedian describes the weather in New York. I have never been to New York but what he said applies to Beijing just as well:

"Damn, I am freezing my ass off here, this is what it would be like if hell froze over?hey wait a sec, that's nice?what the hell?! It's too damn hot now! Man! Ice, I need ice?I remember when the weather was just perfect here, it was a Thursday?"

Beijing's summer and winter are both extreme while the spring and the fall are very short. Winter is cold and dry (beware of static electricity, it happens all the time especially when you wear wool). Since I am from Israel you and I might not share the definition of cold. When I say cold I mean subzero, under minus 5 degrees during daytime. Usually its snows at least once or twice and you can go ice skating at HouHai Park. Since the winter is dry you get a fair amount of clear blue skies.

Summer however is both hot and humid and when I say hot and humid I mean Israel 'hot and humid', the only difference is that it rains here during the summer. In fact you can feel the humidity rising and building up until one day it just starts raining out of the blue (or out of the gray to be exact). In the spring there are dust storms, though I still haven't seen one here.
All in all, Beijing weather is nice especially in the fall. You just need to understand how it works. The key word is cycles - nice weather> than the sky gets more and more gray "rain / winds"; nice weather again. The thing is a complete cycle can last more than a week so you might have to wait 10 days for a beautiful day with clear sky.