I've used the internal beta of Minty and it's really nice. I'm not a Yahoo! Mail user but it's drastically faster then the current version (Candygram). It's also loads much faster than GMail (something the GMail team really needs to work on). It's heavy JavaScript but uses some progressive loading that makes it much snappier than it's predecessor.
Not sure if it's too late for Yahoo! Mail though. This update is long overdue.