When I'm not driving my car to Kingston I've always used Knutsford, as they were fabulous in the beginning with their new services. But recently, they've been unreliable...they've been late, cancelled routes at the last minute, etc.
If you're on a tight/particular schedule or planning to take Knutsford to the airport...I wouldn't recommend them.
If you can leisurely travel without worrying about time constraints and able to reschedule unexpectedly if needed...they're still a great cost effective and comfortable bus to take.