Thought I would share my thoughts on the Diplomat Aero after owning and using it daily for ~2 years....
I originally bought the pen because it was beautiful and had decided I needed a cartridge pen with pull off cap for work. Cartridges because we don't have fixed desks so keeping a bottle of ink is difficult and filling a pen half way in a meeting doesn't work too well. Pull off cap because I would rather have people be interested in fountain pens and pick it up than try and make sure I supervise any contact.
As reported in other reviews the nib is simply superb - in my limited experience it is the smoothest and most pleasant nib I have used. I would love to have a slightly finer option but that doesn't inhibit the sheer joy of writing with this nib.
I don't molly coddle the pen but don't mistreat it either - it lives either in my shirt pocket or in my computer bag and is my regular user at work - so far no issues with the finish deteriorating or even the silver writing coming off.
As soon as I started using the pen I was having issues with the section coming loose and the feel/sound of the metal threads had a similar impact on me as fingernails on blackboard. So wrapped some teflon tape (plumbers tape) around the threads and that was fixed - in addition it meant the section didn't loosen in normal use either.
and here is the big "but" .... when I received the pen the cap was quite tight - possibly too tight - but over time this has become very loose. To the point that I have often pulled the cap off and left the pen behind when pulling it out of my laptop bag. Also, per the photo, there is noticeable gap and wobble between the cap and body which you can feel and see when handling the pen. It destroys some of the beauty of the pen for me. I must admit this has resulted in me using the pen less often than I was as I can feel it every time I open the pen and have had the cap come off in my bag more than once know.
I have considered leaving it on my home desk as I still love looking at it and writing with it.
hope this is useful.