I have used Thunderbird and Lightning now for many years, and I am very happy with the functionality of both. I get even happier once I see how much Outlook would cost me.
However, the one drawback that I am still struggling with is synchronisation. I have sort of figured out how to write to Google Calender, but this only works well when you are online. What I have not managed is to sync with my phone (Nokia 5800) which only supports Outlook and Lotus Notes.
Converted my old office YEARS ago to Thunderbird, such a great Idea.
Some of my new clients I am putting on Live Mail if they don't want to go the Thunderbird route.
Again Great Stuff! Go Thunderbird!
I tried going the Thunderbird route for the office, but copying and pasting excel tables into emails didn't work (something which happens a lot). I couldn't find a simple way of doing it so unfortunately replacing Outlook was canned. If someone comes up with a solution it will save a lot of money only needing a basic version of MS Office, without Outlook.
Windows live mail... has calendar, a nice search feature, and synchronizes online so you don't lose stuff when you reinstall. I dig open source, but see no point in changing a simple mail client.
Been using Eudora since the year dot and still use it today. I was not impressed by version 8 though which is now open source and 7 has problems with Win7 unless you tweak its default saving path.
I use Thunderbird for personal email, Outlook for my job and Windows Live Mail for my wife's company account. The Microsoft products just seem to have better / easier to use UI's.
I am still waiting for the day that an email client can match the functionality and flexibility (let alone the pure simplicity) of Eudora.
If something can match it, I will switch as Eudora is no longer under Qualcomm ownership and no longer being
actively developed (Ver 8 is now basically Thunderbird with a modified GUI).
I am yet so see such a client.... and I've tried many, including almost every version of Thunderbird!
I'm using Eudora 7.1.0.9 on Windows 7 WITHOUT any issues or modifications.