Your sandisk card must have a white serial number on the back of the card (opposite end of the holes for the pin). This number should begin with the letter B, if it has neither, it is a fake.
I use the Extreme card (8gb, not extreme III) and it is great for writing large files and transferring several gigs worth of stuff over to the computer. But it is also worth having a good CF reader to do the transferring as you will not get the full potential of data transfer with a bog standard reader.
I have the Lexar UDMA CF & SD card reader (model no. RW035-7000) and it works a treat with all fast cards.
I also own 300x Lexar CF card & a 266x Dane-Elec card (bought from 7day), Dane-Elec is a great option if you are on a budget, very reliable at a fraction of the high brand name cost.