I spent some time in September in Beijing. It rained almost every day and at night I could actually clearly see the moon. Never saw a blue sky mainly because of the cloud cover. I enjoyed it though. The climate is bearable at that time of year.
I have also been there in mid summer and it can get really hot and sticky but not as bad as my native Oklahoma. The air is really hazy in June. Be prepared to be drenched in your own sweat.
Tons of friendly people on the sidewalks and in the malls. Be careful because there is a scam behind a lot of the friendly faces. If you meet a pair named Sherry and Amanda ,,,, RUN LIKE HELL!
American food is not hard to find. Just ask a local. A lot of folks speak limited English and most are friendly enough to give you directions to your destination.
There are subways that will get you to most of the big business districts. Taxis are hard to flag down during any time of day. It is just a matter of luck. I never tried the bus because I could not read the bus schedules.
There is plenty to do to keep you entertained and lots of tourist attractions.
Stay away from the Pearl Market unless you are curious and like giving away your money. Everything I purchased there has made its way to the trash bin in just a few months.
I stayed near the forbidden city and transportation was good there. I walked to my destination most of the time.
If you can afford it get a local translator to help you out and get you settled. You can find a lot of local help here at this website. Just type in the city of intrest to get a list of local folks to assist you.
http://www.localyte.com/travel-guides/China?&sq=beijing&x=7&y=11