I love my 555R's. These are not an everyday tire, nor are they a track only tire. They are for the car that see less than ~5000 miles per year and gets run at the track and street for fun. They aren't as sticky as the other DRs, but you could get 10k miles instead of 2k miles, like the other DRs. Of course, your mileage could vary, and it just depends on your driving style and how many passes you make at the track.
Thus, they are a perfect fit for my 88. I drive 2000 or less miles per year, run at the track several times per year and drive the car for 'pleasure' on the street. I've had them for 2 years now, and I bet I can get another 3-4 years from them at this rate. At the track, I run high 1.7 60' times, with few traction problems (on a 245-45-17 none the less). You have to put a lot of heat in them to make them sticky, and they seem to work best with automatic cars (less shock load). On the street, any other standard radial tire is pretty much useless on my 88, because they just spin until about 50-60mph. The nittos just hook.