I'm a Cingular family plan subscriber.  I just got back from a vacation
where we used pay-as-you-go Vodaphone Italy SIM cards in our Razrs.  I
was frankly astonished at the difference in service:
- V.I. has capability for multiple lines, the SIM card comes with a few
utility applications, and also has pre-installed contacts with Vodaphone
service numbers (both voice and text).
- The call quality was SIGNIFICANTLY better when calling to the U.S.
than Cingular within the U.S.  People I called on U.S. landlines thought
I was on a landline (which is never the case with Cingular).
- The connection time when calling the U.S. was immediate when placing a
call - no more than one second, compared with Cingular taking about 10
seconds within the U.S.
- If I used my Cingular service to call Italy from the U.S. it's
$1.49/min to landline or $1.75/minute to mobile.  Vodaphone Italy to
U.S. was .55E regardless of line type
- Had I turned on Cingular International Roaming I would have paid
$1.29/min calling within Italy. Vodaphone was .20E/min.

The price differences are even more striking when you consider that the
Vodaphone per minute rates are HIGHER for pay-as-you go customers.
Also, I found it odd that my husband's phone (a sub-account on our
family plan) had Cingular international roaming activated by default
when my (master) line didn't.

If you're travelling to Europe and don't need/want to be reached on your
 regular cell#, get your phone unlocked and go with Vodaphone!  (I'm
also sure there were a lot of other features I could have used but my
Italian is not good.)