Why switch? I'll tell you why I switched from Thunderbird to Claws.

I got sick of Thunderbird's pathological memory leaking. I start by email client when I log in and leave it running indefinitely (days-weeks). Thunderbird continually started out at 1 GB but grew over time without limit, even if only sitting idle, collecting mail. After a couple of days it would be creeping north of 6 GB. Several times it drove my PC into pathological swapping, making the PC impossible to use and extremely difficult even to get control of to shut down.

Claws is currently using 0.1 GB, even after running for a week.

I used Thunderbird for a long time and then switched to Sylpheed. For one its about as good an email client as Thunderbird. That's all I really used TB for. If I want to use a calendar TB is not my first choice. I would rather use something along the lines of a project manager that has gnatt charts etc. So for email Sylpheed is elegant and lightweight and focused. I check my inbox and get my mail done then close it down and get back to what I was working on. Where as with TB I constantly had it open in the background and ended up messing with calendar and to-do more or less procrastinating.

Super fast and lightweight. That's what I liked about it.