I'm currently using a pair of Belkin 200Mbps adapters to connect my upstairs network switch to the one downstairs.
The main issue I had to start with was finding wall sockets that gave the maximum throughput and this varied a lot from one to the next. With the sockets I'm using at the moment, I'm effectively getting ~3.5MB/s (~28Mbps.) With the worst socket in this same room, I got ~1.5MB/s (~12Mbps.) These also seem to work fine in the basic 4-socket power strips, but they do not work with any type of surge protected strips.
For reliability, I've never had a dropped connection caused the link that I'm aware of and I find them far more reliable than the last couple of Wi-Fi access points I've used, especially for streaming TV. With Wi-Fi, I was unable to watch realtime streaming broadcasts without regular drop-outs, where as these stream fine with the Powerline connection.
Basically, Powerline Ethernet performance will depend a lot on your home's wiring. For streaming internet, as long as the two pair up, they should be more than adequate for your cable TV on demand service.