Point Number 1: Make sure the wider rim fits your bike. And consider that the rim and your bike (yes, even your bike) will flex a bit under hard sprinting. Will it rub? It will on many frames. I can get it to work on my Cervelo Soloist Carbon, but only up to about 800 watts (that's as hard as I can go and still look at the back wheel). I can feel it rub on a good hard effort. OK for a time trial, but not optimal for a road race or crit. Now this isn't HED's (or Zipp's) fault, it is the bike manufacturer's, many of which have fallen behind the wheel makers' trend to wider rims for better aerodynamics, strength, etc. The rim does not flex any more than other quality rims I have ridden (again looking at my back wheel). There just isn't any room for more than 1.5 mm of combined wheel/bike flex due to the wider rim profile. The Stinger 6 is stiff enough for Cav (at least for a front wheel), so they are stiff enough for me. (note: I contacted HED and frame flex causes chainstay rub, wheel flex causes brake rub. Makes sense if you think about it).
Did you notice that Cervelo increased the chainstay width on their premium S3 road frame for us mortals for 2010? That was done in 2009 for the pros, I suspect to allow them to use the new wider Zipp wheels on the tour. Trickle down from the test team is working (but not fast enough). What is the point of extremely aero time trial frames that don't work very well with the most aero wheels on the market? I'm sure it is just a question of time until this is rectified, but in the meantime do you want to buy an S2, P2, P3 or P4? UPDATE 13 Mar 10: Went to the LBS today with my Stinger 6's in hand and guess what? THEY FIT THE P3! To be fair, many other manufacturers have the same issue. Check before you buy.
Point Number 2: The wider rim may not fit your brake calipers either. The wheel does flex more than 1.5 mm at the brake, but that is easily remedied by opening your brakes as required. Assuming they open up wide enough. Doesn't work with SRAM or Zero Gravity - the rim is too wide. Once again, the manufacturers of other components need to catch up with wheel guys. Works fine with Campagnolo and I hear it works with Shimano but I haven't tried it. Those brakes have a wider maximum width capacity.
Point Number 3: I think the wheels are fast. Now I had some pretty good results this year. And my good results were coincident with the implementation of these wheels. I was working on my time trial position at the same time too, so there is a confounding factor. Nevertheless, I squeezed out a couple of extra km per hour at what feels like the same effort (and power measurement).
Point Number 4: The wheels perform well in cross winds as advertised. The very first time I used them, we had an extremely strong cross wind. It was just a club time trial, so I thought: What's the worst that can happen? I was able to ride these wheels when everyone, regardless of wheelset, was complaining about the wind blowing them off the road. Sure, I wobbled a bit. So did everyone else. But I stayed upright and didn't veer into traffic or the ditch. I did find that on a downhill, I had to back off a little bit. At speeds of 50 or 60 km/hr, I did not feel comfortable that day. But on every occasion after this, the wind had no effect on me.
Point Number 5: The wheel graphics are lame, but they peel off cleanly. I prefer the stealth look over HED's graphics. Plus they don't pay me to advertise.
Bottom line. I like the wheels and I like the price point. They are significantly less expensive than Zipps. HED compares wind tunnel performance of their wheels to Zipps, and the HEDs were better of course. Regardless, HED and Zipp share the patent for this wheel design and I suspect there isn't a lot of difference between them in a real world ride. So, being a (cheap) rebel, I opted for the HEDs rather than the more popular Zipps. I don't regret the purchase.