The Web site support is good, but it would be much better to have a dedicated application for Windows as there is for Mac. Background synchronization via iCloud and Cloud 7 is, of course, critical.
After two years, I finally went to a different product. But I hope Appigo does, someday, make a solid Windows app. I liked Todo, so I'm open to coming back!
Yeah, I really like Appigo's iOS (and Mac) app, too. A big reason I like Todo is because the tasks have lots of attributes. Start Date is important for me and Todo implements it well.
I am now using toodledo which also has start date. A big pro for toodledo is their powerful "make your own view" which is called "search;" and a search can be saved. In my opinion, toodledo is the only app to truly allow users to [almost] "make your own system." Because of the power and features, though, it can take some learning to discover that toodledo might function exactly how you want it to.
For desktop, it is only web-based, which might be a disadvantage. Toodledo's iOS app is nice though. Another big con (to me) is toodledo does not have drag-to-reorder tasks (which Todo does really well).