Betterbird: A Thunderbird Fork

Just came across this, and not sure how to feel about it.

The blatant marketing-speak and dissing of Thunderbird was a bad first impression. But the problems they mention with Thunderbird do fit in with the attitude that current Mozilla takes with products, and may be indicative of further Apple-isation of Thunderbird, in which case this would be an essential alternate.

What do heavy Thunderbird users think of the problem (if there is one) and solution (which this claims to be)?