Well, I'm not even sure where to start. I can't even show you a screenshot of the standard UI, as mine is so modified. This is an old pic showing an early modified UI with the 'Das Boot Officers' mod. The original faces were from in-game officers.
A good place to look would be to take a scroll through the 'Unofficial Official' screenshots thread. That will give you an idea of what we see everyday. It's more than just the graphics though. The game has been modded for five years now, and it has reached its pinnacle. And yet people are still making new mods for it every day.
These are some shots of one of my personal favorite mods - FLB Sales's Simfeeling Six-Dials mod.
What it does is remove everything from the screen and turn it all into slide-outs, so you don't have to press NumLock+Del to enjoy the view without stuff all over. Other mods do just the opposite - put all sorts of dials on-screen to make manual targeting easier.
All I can really say is that it's about how the game makes you feel, and SH3 lets you feel like you're really there, whether your in a Type II in 1939 or a Type XXI in 1945. I guess what I really like about it is that it is complete, and fully playable. I know that sounds like a slam on SH5, but I also play SH4 just because it has several improvements over SH3, but hasn't had as much attention from modders, and I'm looking forward to the day SH5 is ready to replace them both.
There's not really much else I can say about it. It's just that at this stage it gives me more of the experience than anything else.