I was an early dopoter of @home. I've used mt 8320, T409 and Katalyst to make uma calls from multiple locations in Europe, Asia and South America. There is no "international roaming" charge.
With the 8320 you can log in on a separate access page w/ wifi, sign in and uma works. With the T409 and Katalyst you cannot do this but you can sign into wep encrypted connections.
As for Japan, just last week I made uma calls from the IFCL in NRT.
When @home first came out there where definately bugs especially logging in. In the lst 4 months I have not had any problems and the bugs seem to be fixed. It is my understanding that a few routes have the specific ports blocked by the admin and TMO can do nothing about it.
Another solution that I think delivers a clearer, more stable connection is
www.truphone.com. It uses a different technology, SIP I think, which is like a voip enabled cell phone. There a very few phones that have this ability. You can view the enabled phones on truphones web site. I use the N95 8gb and it is better than my landline.