There is a known hardware bug affecting every PSU and cards in the 6500/7600 series. It manifests as a significant chance of permanent failure on power-up after being on for several years. As long as the chassis stays powered everything is fine. If you reboot or power cycle you may end up with some random dead cards or power supplies.

The use of the "enterprise" 6509 as a backbone ISP router was the worst kept secret in the industry in the 2003-2007 timeframe. During that time you would see about about 60% of the routers in a carrier hotel were 6500/7600's. Most of the rest were Juniper MX or M with very few Cisco 12k, ASR, or CRS. The 12k used an inefficient fixed cell size architecture and focused on ATM line cards and the CRS1 was far too expensive for most operators. The Ethernet-based 6000/6500 series was originally developed by the enterprise vertical at Cisco as an L2 product, but it gradually evolved to full route scale L3 features at a much lower price point than the "real" ISP routers. This price advantage was especially dramatic when 10GE interfaces for the platform came out in 2003. Cisco replaced the 12k with the more popluar Ethernet focused ASR series but the very low cost of the 6500 kept them in use much longer than they should have been.

While dirt cheap due being officially marketed at enterprises and a robust greymarket for parts, the 6509 was far from an ideal platform for ISP's. It generally performed well but there were a number of quirks from an internal architecture that gradually evolved from L2 only->full scale L3. RP cpu/ram was woefully under powered and control plane policing was marginal at best. Any significant packet flow to the RP instead of being forwarded by the ASIC hardware would instantly cripple the router and cause it to drop all BGP/OSPF/IS-IS sessions. In addition to broken control plane policing there were more scenarios where packets could be punted to the RP unexpectedly compared to other platforms. For example if any of the TCAM partitions filled up then ASIC forwarding was immediately disabled and ALL packets were sent to the RP! You also had to watch traffic levels when round-tripping on adjacent 10GE ports on the most popular cards to ensure it didn't exceed about 6 gbps or those ports would start dropping packets. Real ISP routers can often do full line rate in+out on all ports.

The monolithic IOS for the platform was notoriously buggy, partly due to the architecture and partly due to an enterprise feature set that most ISP's didn't even need. It was wise to avoid any version where the release notes claim to have just fixed a catastrophic bug (you wanted to see only boring fixes to cosmetic bugs or features you would never use) and to search the c-nsp mailing list for other's experiences with the version. Otherwise you were playing "IOS Roulette". Because of this it was common to run a known stable version much longer than normal, running the same version for 5+ years in some cases. After the fail-on-power-up bug became known most left them running without even a reboot until they could be replaced.

In more recent years Arista has made a similar play by evolving their switches from originally focused on minimum L2 latency to full route scale L3, and more recently adding full MPLS support at a lower price point than the equivalent C/J products.