This was on their website FAQ:
"We have found an incompatibility running Signature under iPhone OS 3.0. If you have upgraded to OS 3.0, pressing Test Hot Link in the Edit Hot Link view may cause the application to quit. We have submitted a fix and waiting for Apple approval."
I also read the reviews and one person said it crashed when they tried to use it. Maybe that is just one particular process that you have not done tried yet?
The app lets you create these little "hot links" under your signature. So, for example, if you use Facebook or Twitter, you can add these little Facebook and/or Twitter icons under your signature that, when clicked, take you directly to your Facebook or Twitter page. When setting up these "hot links" you can test them from within the app to make sure you typed everything correctly. They are saying that sometimes, when you test them, the app will crash under 3.0.
I don't use hot links, so I haven't noticed that. I just use text-based signatures.
However, they appear to only be saying that it might crash when you test it. That doesn't seem like a big deal.
The app has never crashed for me, but, as I said, I don't use hot links.