I plan on competing in Time Trials in the spring and I have built my TT bike and have been contemplating using disks but not sure if it would be worth the investment. I said that about the areo helmet but purchased one anyway. It made a significant difference especially on windy day. What are your thoughts? Right now I am riding 23 miles an hour for 10 miles and 20 -21 miles an hour for 20 miles. Just giving you an idea of my level of training and level.

Here's my thoughts because I'm bored, not because I know anything about the subject...
Here's a random local Time Trial result, I think this is a 20k: https://www.ic3-ntx.org/2011_March_Results.pdf
23 mph would give you a time of 32:24, which in the men's 4/5 would be good for 24th place.
If you could add half a mph to that, for 23.5 mph, that'd put you at 31:44, which would be good for 21st place.
So the question, what's that worth to you?

Realistically, try to do more research and see what that does to your speed. I just assumed the 0.5 mph increase, and don't know what it actually runs.
If you can get results from your local races, that'd be a better time comparison.
Jump over to the "Racing" part of the Road Cycling Forum, and some of those guys will have all the answers.

I appreciate the input. I have about eight months and I think I can get it up to 24 mph for 10 - 12 miles with hard training and I have heard mixed thoughts about the discs.