Cisco

Pros:

They're well-made, reliable, and tend to function for a very long time without going down.

Well supported--it's very easy to find documentation on how to do something, or a company/person who can do whatever it is you want done for you.

Very large feature set. Cisco offers numerous open standard and proprietary protocols and options in their code that other vendors lack parity with.

Stable code and quick bug fixes. Bugs that cause major operational or security problems have been extremely rare, in my experience.

Most products have a CLI that is easy to use and understand.

Cisco's Price. Sometimes I think Cisco should be spelled Ci$co. Complex licensing. One could invest as much time learning their licensing schemes as they would studying for a CCIE. Inconsistencies between products. Cisco has acquired numerous companies and incorporated their products into Cisco's portfolio while retaining much of the code base--this leads to many quirks when working in the CLI's on multiple products. Most GUI software is written in Java and can do a lot of dumb things--specifically the ADSM software used to configure ASA's. Occasional interoperability issues when connecting to products from other vendors. Software releases can be a bit confusing, as figuring out which features you're getting is not always clear.