Thanks for writing in. I've worked with HP switches in a number of environments and I can say they are great switches. I have not worked with their routers.
The primary difference between the two is price. The HP switches are typically half the price of their Cisco equivalents. For access switches, where you need high port density, that equals a huge cost savings. However, HP switches are less expensive because they don't have the robust feature set a Cisco switch has. So if you need advanced routing, like policy-based routing, you would want to go Cisco.
A common deployment that I have designed for many companies is to use Cisco switches in the distribution/core layers and HP switches in the access layer.
There's plenty of other great switch and router vendors out there. Extreme networks makes some very flexible layer 3 switches. Juniper makes amazing ISP grade routers and has a full line of enterprise grade switches. 3com, Adtran, and Brocade are also big players in the enterprise switch/router market.
Hope that helps,