IntelliJ is the leader when it comes to Grails 1.0.x development, but its support for Grails 1.1 is a bit lacking.  This lack of support is what inspired me to give TextMate a try.  I've been using it for over a week now and have documented some of the pros and cons.

The Good

There's many things to like about TextMate.  Here's what I liked, in no particular order.

1. Grails 1.1 support
   Grails 1.1 is fully supported.  Grails commands may be run from within the editor or the command line.   The only catch is that TextMate uses your environmental settings, so Grails needs to be configured to run properly via the command line or you may be in trouble.
2. Resources
   It's very refreshing to work with an IDE that doesn't require 500+ MB of memory to run.  All projects I worked on within TextMate maxed out around 40-50 MB of RAM which leaves plenty of extra RAM available for other applications to use.
3. Easy Configuration
   Configuring a new Grails project for use within TextMate was an incredibly liberating experience.  I still occasionally have nightmares about trying to get the impossible legacy Java project to work properly in IntelliJ or Eclipse, but Grails has simplified this process quite a bit, and TextMate has simplified it even more.  All you need to do in TextMate is drag the root folder of your (already created) Grails project into the project window and you're done.  It doesn't get much easier than that.
4. Battery life
   This took me a bit by surprise.  TextMate is much more CPU friendly than IntelliJ.  This decrease in energy consumption is very noticeable when running off of battery power.  My battery life increased by 30 minutes when using TextMate exclusively.
5. Fast
   TextMate starts up in seconds and is very responsive.  IntelliJ can take minutes to launch.

The Bad

With TextMate you get speed, simplicity, and good Grails support, but you have to give some things up as well.

1. Code completion
   I miss code completion.  I never realized how much I leaned on it when working with classes and libraries which I was unfamiliar with.  TextMate has a poor man's version of code completion, but it's not as easy to use or as intuitive as that found in IntelliJ.
2. Click through
   This is the ability to hold down the command key and click on a class name within the editor and have the editor open up that class.  There is no click through support in TextMate which can make navigating around a project a bit more painful than it has to be.
3. Method list
   I like to be able to view a list of all methods within a class.  I haven't found a way to do this in TextMate, but I'm hoping a bundle exists out there somewhere which will provide this functionality.
4. Debugging
   Ah, debugging.  A very useful but often abused feature included within IntelliJ.  If you want to debug in TextMate, you'll need to do it the old fashioned way with logging or print statements.
5. Refactoring
   Other than the ability to find and replace, refactoring doesn't exist in TextMate.
6. OS X only
   TextMate only runs on the Mac.  Windows, Linux, and OS/2 users are out of luck.

TextMate is a very capable IDE with good Grails support.  It must be stated that TextMate provides support for most popular languages and even some not-so-popular ones.  TextMate really shines in its simplicity and ease of use, but it is lacking in many features which come standard in the more popular Java IDEs.  Another knock on TextMate is it costs $50 which is $50 more than NetBeans or Eclipse, both being more full featured IDEs.

I'd recommend downloading TextMate and trying it during the 30 day free trial.  It's great for small projects or demos.  Due to its lack of navigation 'perks', I feel large projects quickly become unwieldy, but you may find you prefer it when working with a smaller codebase.