I started using Ardour3 almost as soon as it was out because I liked the
improvements, but I also found it pretty unstable and was constantly
saving snapshots because it was crashing very often or losing the undos.

Now I'm using Ardour4 and it seems MUCH more stable.
I think the improvements over Ardour2 are HUGE, just a few examples off
the top of my head, excluding MIDI:
- the grid interface for the audio connection manager is the best and
  most usable I've ever seen, in any software
- the K12/14/20 meters are invaluable for me (I was using jkmeter for
  that before)
- I can configure the meters to go red at, say, -3dBFS instead of 0dBFS
- the vertical scrolling is really good now
- the summary view
- the plug-ins interface and management
- the pan controls
- the stacked view for overlapping regions is a sanity-saver sometimes
And of course, all the fixed bugs.
I'm sure that there are many more and even more outside of my workflow,
and I don't even remember Ardour2 well.

Is it perfect for everyone? Well, nothing is, I work daily with
Pro Tools and it gets its fair amount of cursing from me.
But I think that Ardour has never been so good as now, and even people
who are happy with Ardour2 and didn't like Ardour3 should try Ardour4.

As a side note, I know there's people that still uses Pro Tools 5 (now
it's at 12!) because it works and a messy upgrade can be too risky if
your income depends on that, so I know the concerns.
Paul, please be aware that people that uses software to work can be VERY
conservative, I think it's at the very least understandable and that
it's a choice that don't deserve to be insulted, especially when the
transition to A3 is not been so smooth.

-- Emanuele Rusconi