These modules are ahead of their time. The biggest limitation to their use are not the modules themselves, but the environment of most users. When running on 32-bit Windows you will not be able to utilize all 4 GB. When running on 64-bit Windows you may not have many applications that are compiled for 64-bit. On the other hand, users of Linux can definitely look forward to having more memory. With 4 GB kits becoming more and more available, this means that you can now build servers and workstations with 8 GB, without a lot of added cost, for FB-DIMMs for example (2x 2 GB FB cost 700$). Also you do not need to buy expensive server boards or server CPUs.
As expected the memory did not overclock much, but overclocking may not be that important for people who need 4 GB. When the system runs out of memory during memory intensive operations, it will start to swap to disk, which is more than 50x slower than the main memory. So just by adding more memory you could improve the speed of your memory-bound applications by the factor 50.
More memory may also be a good investment into the future. A fresh Windows Vista install will consume about 650 MB of memory during booting, without any applications running.