Every day, I have to work with two applications developed this way, with the "Javascript UI" pattern. It's the most frustrating period of my day, bar none.

When you build one of these things, you spend most of your dev time running it locally, and as a result it's snappy. You can do things like loading a blank page for your task list, then flowing in tasks via Ajax and it will render pretty much instantly. It's so fast that you'll even consider loading those tasks one at a time.

For the rest of us, using an application like YouTrack or Flow (getflow.com) is sheer torture. View an item, hit the back button, wait 10 seconds for the list page to come back up, reload its content, and rebuild the page before your eyes before you can even start trying to scroll back to where you were.

Compare that to the old user experience where that list page would be a chunk of HTML generated at the server in 50ms and handed to you across the wire. Your browser would cache it, so the back button operation I describe above wouldn't even involve a trip to the server most of the time. Your browser would remember its previous scroll position. It was fast and it worked. It never occurred to any of us that people would decide to throw it out and replace it with something worse.

So if you're considering building one of these things, step one needs to be spinning up an EC2 box on another continent to act as your "dev" server. Latency is suddenly your number one priority in life, with your app being secondary. Please don't complain about this fact, since it's something you chose for yourself.

Thanks.