I have a dual Celeron system, I do not find any great improvement compared to a uniprocessor system. Anyway, most programs are single threaded, therefore, it would not benefit the SMP users out there.
The benefit of using SMP system is, you can have more things done at the same amount of time, not you can have more things done in less time (which many people misunderstood). It can only show its potential when the program is multithreaded.
I do agree that Celerons are crippled by its low amount of cache compared with PIII or Xeon. But when considering the number of Celerons you can buy when you buy a Xeon, I think Celeron definitely win the performance/price throne.