A friend recently bought some 18650 Ultrafire cells from Ebay, these are 18 mm diameter by 6.5 cm long and Lithium Ion. The capacity on the label is 4000 mAh which seemed optimistic for their size.
In the past I had purchased Ultrafire / Trustfire cells from Ebay with a stated capacity of 2000-2500 mAh and found them to be rather overstated in capacity. These cells were used for the FT-817 a couple of years ago.
For the hell of it, I put one of these 4000 mAh batteries on a Cadex 7200 battery analyser. After a few charge and 500 mA discharge cycles, the capacity settled at 27% of the stated 4000 mAh, and the internal resistance was around 300 milliOhms. This has the real capacity at 1080 mAh at a 500 mA discharge rate. The other cells were close to this capacity as well. <update> One of the other cells measured only 240mAh capacity from new!!!
So advice is to steer clear of these cells if you want the stated capacity! I have heard that the Panasonic and Samsung 18650 cells live up to their capacity although I wouldn’t be surprised if there are counterfeits of these out there!
I_have some Sanyo branded Li-Ion 18500 cells (18mm diameter and 5cm long) that are labelled as 1500mAh. These cells actually slightly exceeded their stated capacity under a 500mA load.