The main improvement here is speed!
QMC sampling (i.e. the sampling that is done when MLT is set to false) Is much faster. It should be generally preferred over MLT sampling, unless the scene is so tricky that MLT is needed

Also, Environment map sampling is heaps faster, as you can see in the images below.
Finally, the GUI itself should be somewhat more responsive, especially if you set thread priority to below normal.
The network rendering has also been improved, see the change log for more details.
I'll update this post with some concrete speed-up factor numbers later.
This is a Beta release.
If you spot any bugs or problems, please make a post about them in this thread.
Thanks!
Indigo for Windows 32-bit:
IndigoRenderer_2.2.1_Setup.exe
Indigo for Windows 64-bit:
IndigoRenderer_x64_2.2.1_Setup.exe
Indigo for Apple OS X:
indigo.osx.2.2.1.zip
Changelog:
2.2.1
* Better handling of tonemapping button presses, to improve GUI responsiveness.
* made tonemapping thread obey low priority setting.
* Fixed mem leaks on scene close.
* Made new 'Updating Image' widget.
* GUI image update period is now read from scene/inifile setting 'display_period'.
* Added EXR image save option to GUI.
* Faster rendering when mlt=false.
* Faster rendering with environment maps.
* Network master now sends messages to all slaves when halt/quit is requested, and waits for final frames to be uploaded
before quitting.
EDIT: the Cornell box scene converges to a perceptually noiseless level approximately twice as fast with version 2.2.1, relative to 2.0 stable.