All of the racing I do is in the form of either bike Time Trials or Triathlons. Both events (depending on course and conditions) lend themselves toward aerodynamic efficiency, since the focus is on individual performance rather than pack/team strategy. Without a peloton to protect you from the wind, aerodynamics can play a big role in results. Think about the difference in your effort level being on the front of a pack, versus being in the 3rd position or farther back....right, Scot?
The argument for a disc wheel is that is smooths out the airflow across the rear wheel, reducing drag. Also, in some wind angles, it can act as a sail and give a little 'push'. There's also the argument that it streamlines your wallet, thus further reducing drag....or, it could increase drag when your spouse finds out how much you spent on a *single* wheel. The drawback is that they can be tough to control in a strong crosswind....and you'll get laughed at if you use one on your next group ride.
For the State Time Trial championship race down in Pattison, TX this year, I rented a HED carbon disc wheel (for fun, and to see what kind of an benefit I might receive).
The rental ran about $100 for the week for a wheel that normally costs around $700 new. The wheel was lighter than my 60mm carbon Real Design wheel and performed well. Besides, a disc makes a cool roaring sound as you pass someone....probably sounds like 'whining' if *you* get passed while using a disc, but that's a different discussion.
I like new toys for my bike, but couldn't justify spending $700+cassette+tires on a wheel that I might use 5 or 6 times per year.
Enter the Wheelbuilder.com aero wheel cover ($100 delivered). This is basically two pieces of lens-shaped plastic that attach to a standard rear wheel, covering the spokes and creating a 'disc-effect'.
Mounting the cover takes about 10 minutes, requiring the cassette to be removed and replaced (a good opportunity to deep clean the cogs!), and small plastic bolts to be fastened around the perimeter of the cover.
Weight is a bit of a drawback with this product, as the wheel is significantly heavier than a carbon disc (like HED or Zipp), since you have the weight of the (non-carbon) cover+spokes....but still, not terrible. I've done two races with it so far and didn't feel that I was dragging an anchor up the hills. I've also ridden it in 20+ mph winds without experiencing control issues (I have other control issues, but I don't like to talk about it).
Does it make me any faster? I have no idea (but I'm going to say 'yes' since I've already bought it). Physical and environmental condition changes between 'test runs' would probably be enough to skew any empirical results. However, I believe that the mental aspect of endurance sports is significant. Waiting on the start line with a skinsuit, shaved legs, aero helmet, aerobars and a disc wheel means you've told yourself "I'm riding fast today".......and......."I hope I don't get pulled over by the local sherrif and taken to jail looking like this".
In my last time trial race, I beat the next guy by less than half a second. No, it wasn't for a podium spot, and I'm not going to say that a disc wheel gained me "X" number of seconds in that race, but it was still one place higher, and I did beat my time from the previous year by almost 5 minutes. Some of that was better training, strategy and better bike positioning, but I would also include the aero advantage too.
If you're not prepared to spend $700-$2500 on a lightweight carbon disc, the (around $100) Wheelbuilder.com aero cover is a good alternative. Look fast, feel fast, be fast.