Long before I made the switch from Outlook to Gmail, I discovered Google Desktop Search and I was amazed to see how superior it was to the existing Search options on my PC. [Update December 22, 2012 – Google Desktop was discontinued in September 2011.]
Now I've got that same incredible Search performance in my email and calendar. And I am LOVING it.
While I used to create a folder for every client, so I could quickly and easily see all of our past correspondence and conversation threads, this got very cumbersome as my business grew.
Now all I have to do is type the client's name in the Gmail Search box and I get an instant folder (I had tried this in Outlook but found it slow and unreliable).
Or if I'm trying to find a specific email, I can just type a few words that I remember from the conversation, and voila – amazing results again!
I even find myself emailing information to myself with descriptive subject lines and body text, just because I know how easy it will be to find it later!
As I already pointed out, there's tons of storage space in my Gmail account so there's no danger of clogging anything up by doing this (while that was certainly my experience with Outlook).