The questions is, what are you looking to do with the 3G data card connection?
I own a 3G card on Vodafone and I use it regularly. I have never tried to trade from a 3G card but I have tried to play online-poker and occasionally had problems with dropped connections.
I've found the card pretty good for the purposes of browsing, MSN Messenger and email. Just be aware that the quality of the service seems to be variable and if you are used to a broadband connection from home you will really notice the lower speed.
Bear in mind the following;
1. The theoretical speed of a 3G card is only 1/3Meg (384kbps).
2. I formally worked for Bloomberg and we recommended a 2 x 2 Meg line as the bare minimum for anyone attempting to trade.
P.S. You might want to check out the new 3.5G cards... I haven't used one so I can't vouch for it but I've heard good reports.