I bought C3 license yesterday and spent about 10 hours editing a big(ish) project - 55Mb, 1000 events.
I must say, it was not a pleasant experience.
1. It CRASHES!
Chrome is constantly running out of memory and dying with that stupid "Oh snap!" message. This often happens when I'm trying to open a big layout. I have 8Gb of RAM, how much more does it need?
Desktop C3 version is no better.
After I repeatedly lost 20-30 minutes of work, I started saving my project after every change. And this leads to the second problem:
Internet in my country is slow, especially for uploads. Saving 55Mb to a cloud takes 5-10 minutes, so it's not an option.
Saving to a local file is really annoying! You need to press Save, confirm save dialog, wait 1-2 seconds until the file is downloaded, select save location, rename the file, confirm.
In C2 you simply press Ctrl-S and that's it.
3. Keyboard shortcuts.
I've been using C2 for two years and naturally developed a habit of pressing Ctrl-F4 to open Debug Mode or Ctrl-W to close a tab.
I shut my C3 window about a dozen times yesterday! Desktop version can survive Ctrl-F4, but still closes if I press Ctrl-W.
Debug Mode is one of the most frequently used features in Construct. And now it takes 3 clicks to access it!
Menu -> Click -> Project -> click -> Debug -> click
4. Other browser-related issues.
Event dialogs sometimes lose focus. Pressing Backspace sometimes causes Chrome to display "Press alt-left to go back" instead of deleting last character. These things are rare, but still happen.
5. Built-in editor for files is a nice new feature, but some keyboard combinations don't work there, for example Ctrl-Right, Ctrl-Left.
This makes it hard to edit long lists of values.
This is just my preference - I like thin window borders. C3 windows are so bulky...
I really hope Scirra will continue to support C2 and not retire it.
As with all these issues I don't think C3 will ever become a good replacement for me.
EDIT from 23 Feb:
I'm a bit of an idiot.
Just realized that I've been using "Desktop shortcut" created from Chrome and thinking that it was the "Desktop build".
(I did download the Desktop build, but forgot to add a shortcut to it)
Can confirm that the real Desktop build works better and doesn't have some of the issues I mentioned above.