Let's just say that irb is worth learning to use and useful  to know.  
It can be a lot quicker than Creating a new file, changing  
permissions on it and executing it. You can explore side effects and  
snippets very quickly. You can even open a file's contents in irb.  
One thing is for sure, if you have Ruby on a system, you have irb.
While learning Ruby, irb is useful.
TextMate is easy enough to start using, but like all good editors  
there is a learning curve. The auto completion isn't what you might  
expect from a full IDE like XCode or VisualWhatever. But it is a good  
quality app with a small footprint, not demanding of the system, and  
pretty robust.
The cool thing is the way TextMate just implements existing OS X  
software to do things like test run your code or validate or preview  
pages. It's really sort of a testament to the volume of good  
libraries in the Cocoa framework being used in ways they are meant to  
be used.
At first I wasn't sure if it was worth the price, but the more I used  
it, I quickly felt it was worth more.