Tires:
The 615's aren't my favorite, but they are fun, cheap, last a while, and work great at cooler temps. If the track temps are below 75*F, the 615 makes tire noise, competitive amount of grip, and are predictable. Hot ambient and track temps changes the characteristic of the 615 to make no noise, are hard to predict, get 'greasy' within a lap or two, and are not that great to learn on. If you live out here in Southern California where its always 80-100*F, the 615's are often a poor choice. In winter over here, they are awesome, competitive (set a S2000 track record on a local track in winter), and would be my choice for the price.
For the June 2009 issue of Grassroot motorsports, tires were tested in 15 inch sizes on a honda civic on 205/50-15, except for RE01R (195/50-15) and Starspec (195/55-15)
6th - Falken RT 615 Mean time: 35.302 Sec Quick time 35.125 Sec.
5th - Dunlop Direzza Sport Z1 Star Spec Mean time: 35.209 sec. Quick time 35.061
4th -Toyo Proxes R1R Mean time: 34.522 sec Quick Time 34.452 sec
3rd- Bridgestone RE-01R Mean time: 34.478 sec Quick Time 34.430 sec.
2nd- Bridgestone Potenza RE-11 Mean time: 34.266 Quick time 34.238 sec.
1st - Kumho ECSTA XS Mean time :34.170 Quick Time 34.030 sec.
All tires were shaved to 3/32nd and test performed on an Autocross set up.
*** This was done on an Autocross. Autocross tire demands are different from road course. Autocross tires start at ambient temps, have lower speeds, lower loading forces acting on the tire, and last ~1 minute or less. Road course use involves higher speeds, higher loads, longer sustained loads, longer runs (1.5-3.5 minute laps) - for multiple laps in a 10-30 minute session, etc... Personally its neat to see the 'ballparks' of autocross tests, but take it with a grain of salt. 2 tenths during this test could be far greater + or - depending on how the tire acts from said road course demands.
Nitto's NT05 is a great tire/price, but Bridgestone's new RE11 and Yokohama's AD08 are a step ahead. While many other manufacturers either have a great tire or equal to (if not barely eclipsing) the current best tires (including AD07), the AD08 is coming out and should set the bar once again for someone else to try to beat it. Tire wars are fun.