Others can probably "sell" it better than me, but:
GWX won't fix the crew management portion. SH3Commander does that. Commander is a add on that adds many enhancements to your careers and patrols. Besides some crew management options, you can start patrols at sea, enable malfunctions and/or sabotage, realistic career lengths(no more 30 patrol careers), randomize crush depth of your submarine so you never quite know how deep you're able to go, plus many others.
GWX...where do I start?
- Better graphics.
- 16km atmosphere.
- Many mods that come with it that you may enable.
- More ships and aircraft.
- Better AI.
- Campaign enhancements.
The list can go on. It is definitely a huge improvement over stock.
There are other "uber" mods out there that are very good as well-NYGM, WAC 4.0.
Check out the GWX website:
http://www.thegreywolves.com/
Play stock for a little bit to get used to it. That is what I did and I found that I got bored with it pretty quick and upgraded to GWX2.1 pretty quickly.