Although u can put miles into UA program, Emirates own program is better than UA if u fly Emirates a lot.
You will earn 2500/3500/5000 for Econ/Busi/First class bonus miles after ur first flight with them.
You will earn 20%/50%/100% for Econ/Busi/First class bonus miles if book online.
In addition, they have double miles promotion currently running on LOTS of flights.
so a roundtrip from PEK-SFO via DXB in Economy class will earn u 2500(signup)+23954(flying)+23954(double miles promotion)+4791(online booking bonus)=54479 miles.
ALL class(except award ticket) earning 100% EK miles.
You will become Silver with 25K miles flying or Gold with 50K miles and will earn 25% bonus as silver or 50% as Gold.
In term of spending miles, the best way to use them is to upgrade ur Flight, remeber, ALL PAID CLASS can be upgraded with MILES
All information can be found here