I have a Sierra card in my business lap top through Centennial. Slower than cable but faster than dial-up. The short antenna that came with the card did not work to well. I purchased an 11 dbi external antenna from this company. Works great.
11dBi External Antenna- Sierra Wireless 880 881 AirCard - eBay (item 360033042161 end time Oct-09-08 19:21:40 PDT)