I just tried it out on a message a friend sent me a couple of days ago. It seems to work pretty well.
It looks like it uses OCR to read the message and try to automatically enter the MSG ID and Password for you. It got mine pretty close, but was missing two characters from the password. Fixing the mistake was just like editing a normal text box.
Once I had the right username and password in, and hit "Go", it sat on a loading screen for awhile (seemed long, but honestly, probably not any longer than viewmymessage usually takes, and if they're just grabbing the images from there, I doubt there's much they can do about the speed), then brought my message up with text.
All in all, it does what it says. To me, $5 is a small price to pay for actually being able to get MMS images on my iPhone. Weeeeeeeeeeeeeeee!
Their site doesn't say anything about support for anything other than images. I sometimes get audio files and even 3gp videos sent to me, too. I'll have to test that out and see if Fetch supports them. Probably not.
