Here's a little story from one of the developers to demonstrate the value of Helium as an application. The version of Helium described did not have the recently added visualization window which has a major impact on performance (for the time being).
One night, deep into the timeline of the development process, when things were doing what they were supposed to do but were being visciously slow, I whipped out a profiler (Mike's Java Profiler in this case) and did some exploring. The type of the Entity Identifier, a String, was slowing things down a great deal so I did some diddling to change it to a long. To see if it made a difference I loaded a bunch of mail into Helium.
The change must have been effective because somewhere around 5000 or so messages, performance was still cranking and I happily started to play, seeing what Helium could do. In those 5000 or so messages was a mix of mail I had sent over the last couple of years and mail from a variety of friends, family and co-workers.
Digging around in the mail, performing text and metadata searches, was a revelation. Without the imposed constraint of folders and with the easy, mouse-initiated searches, long threads of connection in the messages, previously undiscovered, were revealed. The simple act of seeing what mail showed up on the same day as particularly good or bad news provided insight, shifted some memories and resolved some questions.
These sorts of things are possible in many mailers, but in Helium, that night, they were right there, easily available and natural to use. It was fun.