I've used a (non-3G) VMC card for some years - but only as a last resort. The software is not terribly stable and even trying to connect can be a real hassle. It often refuses to display any networks and I then have to remove the SIM card, put it into my phone and force it to connect to the local Vodafone network manually to wake the system up. Then back into the VMC card and it's probably OK.
The sensitivity of the little red pop antenna is very poor indeed.
OK - let's say you've managed to persuade it to connect. You never get anything close to the alleged maximum GPRS connectivity rate - and sometimes the whole thing freezes the computer, needing a Big Switch disconnection.
If you remove it without closing the application, you can get the blue screen of death and all sorts of scaremongering MicroSoft geek speak about 'serious errors' etc when you reboot....
The basic idea is good. But it's expensive, not terribly reliable and uses immature technology. The 3G version is enormous!
By far and away the best solution is to use a WiFi hotspot instead! I only ever consider using the card to send e-mails. But due to its slow connectivity, it might be necessary to access a webmail server first, to check that there are no huge documents lurking in your server inbox.
As for surfing the net - very, very expensive and slow if used abroad!