I have been using this new feature during the last days and apparently it works 85% of the times with Vodafone Spain. It seems that it depends on the cell tower as most of the fails are always in the same place.

Maybe it's just a timing issue, if you add a little delay it could be enough.

What I have observed is that the widget switches from 2G to 3G, and maybe it's better to change to Dual, so if there's no good reception for 3G you could stay connected to GPRS.

Edit: As far as I continue testing percent of success is raising.