I love Fastmail and have been a paid user for many years now, but I hate the new AJAXy interface, in particular the infinite scroll as currently implemented. Slow, annoying, and makes it difficult to reach old emails.
This is from the perspective of using Chrome on high end hardware, but I couldn't ask for a better web interface.
That said, they aren't joking about needing a new mobile interface: the current one is usable, but definitely lacks the same polish as the desktop version.