I have a 128MB stick of the Corsair XMS 2400 in my DDR rig. The only thing that I have had wrong with it was that it was faulty when it got sent to me, so I had to send it back. It is rated to run at 150MHz, which it does with ease on my system. The only reason I got it was b/c it is the only CAS2 stuff out there. You can get by with PC2100 at the CAS2.5 and run it at CAS2 without any problems. Not to mention that it is a little cheaper.
I think my memory can run higher than the 150Mhz, but I usually dont run it any higher. I tested it when I first got it to see what it could do. I was shakey at 1.4GHz (the CPU is 1.2GHz.), but ran very solidly at 1.35GHZ. That probably has to do with the motherboard (ASUS A7M266) and CPU than it does with the memory failing.....
If you want it, go for it. Depends on how big of an OCer you are. If you are just an average user that just wants to have a fast system at CAS2, go with the PC2100. You wont have any problems with it. I havent seen prices, and that may affect your decision too.....