I published my review/view of Thunderbird in this overview of alternatives to Outlook that appeared on the CMP online-Pipeline publications, for which I frequently write.
I explain my love/hate relationship with Thunderbird. While I think it has great potential, and I use it as my primary email client, it also has enough frustrating flaws to keep me guessing. But I would challenge you to name a software package you use on a regular basis that doesn't have some aggravating problems or implementation quirks. It's just the nature of the beast.