I have been an ardent user of wunderlist for over an year, switched over from Astrid after Yahoo destroyed the company.
Loved the fact that wunderlist was able to sync over multiple platforms. BUT ... I have had several issues .. like the tasks vanishing, not being reminded properly. No way to set default reminders for tasks, notification widget in iphone doesn't auto update etc etc... I had to give it up
I bought fantastical after being suggested by a friend. I must say combined with the default reminder service on iOS, fantastical has been a really good buy. I generally depend on repeating reminders (monthly and yearly)... and I love the fact that fantastical understands the natural language very well.
Like "Todo credit card bill payment every 1st of the month" ...
If anybody here have used Rememberthemilk, they will understand how wonderful this is.. But RTM is a monthly subscription based service, cannot spend on that monthly. But Fantastical is a wonderful app doing the same thing in a much better way.