This is a great extension to enhance the look and feel of the Lightening calendar add-on - especially if you use or have used Outlook. From my perspective, it provides a professional look and feel to Thunderbird/Lightening with smoother color gradients and various interface tweaks (e.g. calendar tabs). This extension puts Thunderbird/Lightening up there as a contender with the overpriced MS Outlook from an aesthetics perspective.
One issue I have noticed with Thunderbird 6.02 and Calendar Tweaks 3.0 is that it breaks the "Delete" button if you add it to the Mail Toolbar. I always do this as it makes it easier for me when doing bulk activities. If I disable this extension, it works again. I have contacted the extension developer to bring this to his attention. I mention it hear to help others thing they have lost their minds. LOL
Regardless, this extension is deserving of a 5-star rating. It does what it says it will do and except for this one issue I have experienced no problems when this extension is installed.
After further investigation, this reviewer has confirmed that the reported issue is NOT caused by Calendar Tweaks.