As I balance the black-plague of subscription services I pay for each year, a few things are never a contention. Fastmail is one of the darlings of my subscription services. It's a solid email provider and it's constantly being improved.
Today Fastmail greatly improved the mail rules for advanced customers. Previously, creating a custom Sieve rule meant permanently abandoning the convenient rule wizard. Now Sieve scripts can sit alongside basic rules.
Even the basic rules received an improvement. Rules can now evaluate against "any", "all", or "none" modifiers to create more complex evaluation criteria.
The Fastmail blog post describes the improvements in more detail.