Well, Cisco stuff works, there sure is a lot of it around, but in most cases I've seen, Cisco is usually somewhere around third-place when it comes to performance in just about every (performance) category.
Performance is not the reason to buy Cisco.
The size and stability of the company, the broad scope of products, and support are more important in many cases than performance. Most customers I've delt with prefer to have a homogeneous network, managable from a single Management platform.
WHen you go "Best of Breed" for the entire network, you'll probably end up with three or four (or more) management platforms, most usually don't operate concurrently on the same machine (frequently a big Unix/Solaris/HP(S)UX machine.....)...and frequently don't share detail information with the other platforms. When you call support, there's frequently a lot of finger pointing to the "other vendor's" equipment. There may be interoperability issues, the colors might not coordinate...it's hell on Earth...who can live with clashing equipment colors?
Jumiper is a faster router, Extreme and Foundry are higher performing switches, Fore/Marconi ATM was far superior to anything Cisco ever produced...the list goes on...Cisco isn't the best performer....but that 's just not the usual primary criteria for purchasing enterprise/carrier/telco equipment. If it works "well enough," and has the stability and support...coupled with adequate management, it's a better candidate.
And for the record: Extreme also has excellent support, I've heard that Foundry does as well. Fore Systems/Marconi is out of the enterprise network business (screwing untold numbers of their customers, and the people that sold it)...I'm not saying that the other vendors mentioned DIDN"T have great support...what they don't have is the longevity (yet) and the size that Cisco offers.
Getting back to the original question from Muc, personally, I believe that you'd see mostly Juniper routers at the ISP, mostly Cisco routers along the carrier routes.
FWIW
Scott