I am looking into getting in the world of custom rod building, and I am looking into starting with a ultra light rod. It is specifically a 5' Ultra Light with a moderate action. My question is, what will this rod excel at with a moderate action rather than a fast action?

A slower action can help protect light line during last second surges of if you hook into something bigger than the target species.

Colin,

If you haven't already purchased your blank, I'd go for a longer version, I see no advantage to so short a rod.

The longer rod will cast further, make all your fish feel bigger when caught, and better manipulate your presentation over distance. My first was 6 ft. long, I use a 7 ft. rod most often.

Good point ^^

Longer is better.  Hook sets will be much better with a longer rod.  One thing to keep in mind is that the longer rod can tend to feel a little sloppy if made from a fly blank.  If you go fly, 3 wt is about right.  The blank below has received a lot of positive comment on another forum, and is not expensive.  If I were to build a panfish rod it would be my choice.  The extra length even with a fast action will provide some protection if bigger fish are caught.  It is a Rainshadow Revelation blank.

 

REVS72UL-SB

UL Trout & Panfish

7'2"

2-6

1/32-3/16

0.393

5

Fast

UL

1.48

I'm building one on a mhx s720 6' mod,ul.... it's a nice blank 

Surely it is, but for the best ultra light , go longer.

 

Love my Rainshadow XP840, 7 ft. long ultralight popping rod blank. If bass are in the offing I go to my St Croix 5S76MLXF, it has a very ulralight, but XF tip that the butt of a decently stiff Med. Light rod. I've caught trout, panfish, all the bass, speckled trout,  bonefish, steelhead, and many other fish on the St. Croix.