I am new to this forum but I've been a Straightalk customer for over a year now, starting with the Nokia E71 (sigh, I miss that keyboard!) and moving to the Galaxy S2 with the same sim card (I'll be replacing the old sim with one of the "bring your own phone" sims for ATT Straight Talk in about a week).
Roaming with Straighttalk - no worries. I've travelled from one end of the US to the furthest point and wherever there was ATT service, I had ST service. No extra charges. I haven't travelled out of the US with my phone so I don't have anything to add there.
ST settings for getting data and mms to work right.
ST APN settings for ATT and TMobile phones. Link takes you right to the ST page with those APN settings. They work, trust me. I was banging my head trying to get both my mms and data straightened out and working (I lazed around for a long time with the E71 and didn't bother. But the Galaxy? Ya just gotta!). I tried setting advice after setting advice (this one worked for mms but not data, no data except over wifi - urgh!) including a lot of settings advised in this forum. I deleted a LOT of APNs during the process.
So, final victory was found using the basic settings found on the ST page. I only needed ONE APN - not separate data/mms and ST has definitely simplified the info required since I began this process. BTW, these settings are for any ST sim card - just follow the directions for either the ATT sim or the TMobile sim and if you have the old ST sim (found in the Nokia phones) just follow the ATT settings. Now I have mms and data and all is working well. Thank gawd - otherwise I was going to have to run down to BB and get the S3 on their one day sale today and slink back to $100/mo ATT bill.
Good luck and have fun - victory is sweet and I love my cell phone bill;P
Oh, and make sure you reset your APN settings back to the original before you start - cleans everything up and reduces possible headaches.
I also made sure I "selected" the new APN after I created it and then turned my phone off and on again. After that, easy peasy.
