What are the advantages of a billet lower versus a standard forged lower other than rigidity? I can see if you have larger calibers, but the 223/5.56 is a relatively low recoil round.
What are your thoughts?

They look neat.  

The shapes available to billet machining is much more complex and smoother looking than a forging due to the process.  There are certain cosmetic flaws that are inherent in forging metal that can smoothed out with a lot of finish work.  But when building a couple million rifles that are spec'd for a job and not to look pretty, its a cost nobody cares to pay.

Agree with the comment that it looks neat.  Keep in mind when you start expanding an item (normally the billets are beefier looking) and not comforming to the normal shape, aftermarket items may not fit to or around your lower and upper.  I have read post after post about parts that won't fit over the lower and upper.  The choice is yours, but I have to bust out laughing when you start posting "something doesn't fit my billet lower."  What you are paying for in the billet isn't necessarily a more quality or better built lower.  You are paying for the labor costs and machine costs to make the lower.  I'll be honest...The gain I have seen from billet...Not worth the cost.   Also, if you damage or wear one or the other...the replacement cost is going to be the same cost to buy new.

I disagree with the second comment above slightly.  There were only about 4 major manufacturers of lower.  There may be more now.  There are distinct differences between the 4 companies.  Just because it is forged doesn't mean that it's not machined like a billet lower.  It still has to go through a machining process for the mag well, trigger well, buffer, and pin holes.  Some of the manufacturers now put the lowers in CNC machines that clean up the edges and give the forged lowers a more billet looking quality with smoothness.  If pretty is your object, I would put my money into the $100-150 ea lowers and uppers versus the $300-350 ea....I would then put the expanded budget into a matched barrel and bolt, quality sights, and a higher end scope than you would normally look at.  This will gain you the most accuracy for the money.  It's style over substance.  I personally used the $60-125 lowers and uppers.  I have had ZERO issues to this point...Building rifle 5 now.

I bet there will be 20 posts defending their honor and higher expense paid.  Read and believe what you want.  The upper and lower "MAINLY" hold the rifle together by attaching the barrel, stock, and magazine.  The rest is just...Purdy!

There is no advantage to a billet receiver over a forged. Billets must be heavier to give the same strength and rigidity. When a part is forged, the metal grain follows the form. On a billet, the grain runs one way. Billets also cost more money.

Forgings are lighter and cost less. The only reason to go with a billet is to get a configuration unavailable from a forging without having to pay to develop new forging dies for small runs