What icon sets are you folks using?

[Oldsmobile_Mike]

3.9 GlowIcons on my A500
Ken's Icons v4 on my A2000

I tend to spend hours picking & choosing, though.  I'd Imagine I've downloaded just about every icon pack there is on Aminet over the years, and tend to change just about everything to suit my taste.  That's always been one of my favorite things about Amiga - the level of customization - no two will ever look alike!  

For you I would recommend this:  http://aminet.net/package/pix/mwb/StandardMagicWB16

It's a collection of almost every MagicWB icon ever created.

Also don't forget the Peter K. icon library!  http://aminet.net/package/util/libs/IconLib_46.4

PS - personal pet peeve - they're not "MUI icons", they're MagicWB.  MUI was the interface, MagicWB is the icons/fonts/backgrounds stylization.  http://www.sasg.com/  Cheers!  

[Oldsmobile_Mike]

With 3.9 you can just drag & drop.  With 3.1 you'll probably want to use one of the more advanced icon editors to change them out, since AFAIR the one it comes with is pretty rudimentary.

You might also want to take a look at SwazInfo:  http://aminet.net/package/util/wb/SwazInfo18b

AFAIK changing them "en masse" like you're talking about is going to take writing a script.

Take a look at this blog for some other good tips for sprucing up an ECS Workbench:  http://www.mfilos.com/2012/01/guide-making-workbench-prettier-using.html