G.729 CODEC: Worth extra cost?
 

Some of the SIP clients I use have an option to purchase the G.729 CODEC. I was wondering if it is worth the extra cost? Clearly, the server would also have to support G.729. Are there any reliable US VoIP servers that support G.729? I'm curious about the ability to run a SIP connection over T-Mobile's (US) free (albeit slow) international data roaming.

usually you can get quite satisfying results by using the GSM codec if you have a throtteled data connection as well. It has a nominal data rate of 36 kb/s including headers. I think the T-Mobile data is limited to 128 kb/s, so that should me more than sufficiant.
Keep in mind that the latency of the connection is a more severe issue when you are registered to 2G networks.

PS: if you like it scientific i recommend the following read: http://www.sersc.org/journals/IJCA/vol6_no4/30.pdf