It's a complete crapshoot. Anybody who says they can is full of it. Strom, however, is an advocate of caching and thinks caching appliances will proliferate on enterprise networks over the coming years. There is also the problem of user hits not getting to the content providers in a timely fashion. Some caches don't register hits on pages they have already cached, which infuriates content providers.
Most caches underreport.
Most agree, however, that content providers must learn to live with caching at the ISP and enterprise level. The days of direct hits on sites will soon fade. If they mark all of their pages as non-cacheable, it will look to the end user like their sites are performing slowly. Also, the cache could become a bottleneck if overloaded with too many requests and not implemented properly, no matter how efficient the vendor says it is. In many cases, caching is applied as an afterthought, a bonus feature that comes with the proxy servers they purchased primarily to secure their internal network.