Time to look at T-Mobile.
> The prices are great, I need to figure out whether the coverage is OK
> here which basically means whether they let me roam onto Cingular. It
> appears that in some places they do, some they don't.
Generally the way it works is if the home carrier (in this case T-Mobile) offers service in a locality you can only use them. If they do not offer service in a locality often times a roaming partner may offer service *if* the home carrier has a roaming agreement with the other carrier. In any case if there's no roaming agreement you will not be able to register on that network and the only thing you will be allowed to do is to use that network for emergency 911/112 calls.
All carriers including T-Mobile usually have a trial period of 14 days or more where you can try the service before committing to an agreement (usually it's a minimum of a year.) You can try a service out and see whether it works in a satisfactory manner in the places you need it to work (home, work, etc.)