Working fine for me on MightRom FF.
AFAIK, Sprint MMS won't work with the HTC Messaging client 1.5, I really don't see why it would work with 1.8. You can configure it to receive MMS, but it won't send them.
A workaround for pictures is to use sMMS for sending. Or you could install Arcsoft and use it, as you can still access the WM Text App. What would be ideal is to configure Arcsoft for sending, and HTC Messaging for receiving.