Gmail has IMAP support now. It's an amazing tool. As much as I like Gmail's AJAX web interface I prefer a good email client more. There are just things that an email client can provide that even the best ECMAScript user interface just can't match. Email clients have decades of evolutionary refinement behind them. Also, Gmail IMAP means that I now have a way to more effectively back up my 1 GB of email that I've accumulated in the years that I've now used the service as my primary web interface. Up to this point I've relied on POP downloads to keep things in sync between a Thunderbird "hard copy" primary and the Gmail secondary web interface. Now with IMAP available I've decided to retire my POP archive (slightly "cleaner" than my Gmail All Mail, but also dating to slightly prior to my Gmail account) and switch to primarily IMAP access, with some sort of simpler backup process. In light of that I decided to reevaluate my choices in Email clients and see where things stand today. It's interesting too because I have a Gmail account with 1 GB of email and over 80,000 messages in All Mail, a good percentage of which has at least 1 label (and Gmail IMAP duplicates messages in folders for each label). It's a heck of a test case for an IMAP client. I was looking into bigger "PIM" packages because I figured I wanted a good desktop calendar as well.

Outlook is Amazing in an Exchange environment. With IMAP, not so much it seems. I had issues with Outlook freaking out on me. It apparently tried to download every single header in the account from the get go, which is fine, but it had no real status indicator of the process (what folders was it downloading? how many headers did it have left?). When I finally decided that Outlook 2007 wouldn't cut it I found that the data file for the IMAP account had exploded to 1.6 GB, presumably just for header data. It's almost embarrassing. Additionally, on the calendar front, I love the TODO Bar in Outlook 2007, but it apparently refused to display my internet-based calendar as upcoming appointments.
I tried it a little bit, but it just didn't gel for me. Evolution seemed to have many of the problems of Outlook without the things that actually stood out for me as useful tools in Outlook. No offense to the Evolution community, but it does seem like a clone of Outlook's bulk and bloat without the "heart". IMAP support seemed slightly less opaque than Outlook, but had less features for customization and still wasn't entirely obvious what it was working on at a given point in time where it appeared to be waiting for something.
So I ended up back with Thunderbird and it's IMAP support, in comparison, is amazing. The status bar is almost always helpful in showing what's going on. The IMAP account loads and responds almost immediately and only tries to grab information when it is actually needed. On one of my laptops just out of pure curiosity I decided to try letting it grab the headers for all of my labels. Barring what seems to be a bug in Gmail's IMAP implementation (sometimes viewing a folder via IMAP would un-label everything in that folder), I had no problems with doing that and the header data files for Thunderbird topped out just shy of a spry 90 MBs (compared to Outlooks 1.6 GBs). Thunderbird has the best options in terms of customizing how it handles the IMAP account. The only problem... no calendar support out of the box. I had played with earlier versions of Mozilla Calendar and was unimpressed, but Lightning 0.7 is impressive. It's a quick, easy install into Thunderbird and works wonders for what I needed. It even has a bar similar to Outlook's TODO bar that actually displays my events from my internet calendar. The one thing it needs is to show flagged/starred emails as TODO items and it will be even more like the Outlook TODO bar.
I found OfflineIMAP as a simple, easy sync tool to make backups of my Gmail All Mail folder with a decently verbose blinkenlights interface. It's a cute thing to just let run in the background. It drops the synced copy as a simple Maildir format, which is directly readable by a number of email clients and easily convertable into many other formats. Unscientific examinations of the process in progress seems to suggest that disk usage for the Maildir for All Mail is somewhere near the Gmail quota usage estimate.

So Thunderbird+Lightning and OfflineIMAP seems to be a decent collection of software for my needs and I'm really excited by Gmail's IMAP offering because I can use Thunderbird+Lightning on both of my laptops and not worry about sync issues and not be constrained by Gmail's nice, but strict, web client. I have no idea how Gmail can offer an IMAP server with the loss of the potential ad revenue, but I'm happy that they've done it. I make up for it in time spent in Google Reader anyway, I guess.