I have the 6600, not the 6620.  I bought the 6600 (tri-band) unlocked in
Hong Kong (English version).  It's bulky and awkward and sucks battery juice
pretty fast.  I was pretty disappointed in the phone.  Hopefully the 6620 is
better than the 6600, but I would be wary.

I picked up a 6230 in Amsterdam, unlocked (the 900, 1800, 1900 version).
Later I bought an unlocked 6230b from my local "phone pusher."  This is the
phone I am using now (I use the 6230"a" overseas).  It will take a team of
Clydesdales to pull this phone away from me.  It is superior in every way.
Excellent RF reception at both 850 and 1900.  Great sound, great BT
implementation.  EDGE GPRS, MMS, SMS, ABC and XYZ.  Take a look at the
comments posted to Phonescoop.com  Make sure yours has one of the more
recent firmware versions;  5.24, 5.26, or 5.35  (to check press *#0000#).  I
never buy branded or locked phones - ruins the concept of GSM.