Can you only stream from services like Netflix etc.
Both Roku Stick and Chromecast are streaming sticks. The Roku stick comes with a control and when connected has an actual user interface to navigate and do stuff. It comes with apps built in, so you don't need to use a PC/smartphone to make it play; just the remote. The Chromecast is just a streamer and it requires a smartphone, or tablet or Google Chrome browser plus it's Google cast extension installed so you can send videos to it. The casting icon I was referring to looks like this
It's really a painless process. The same icon comes up on my iPhone and iPad. I'm sure it'll look like that on an Android device.
can you actually tune into live TV and stream from the available networks?
AFAIK, the Chromecast didn't have that feature. I am unsure if the Roku stick or the Roku device does live TV. However, I do know that my some apps on my Apple TV 2 allow you to watch some of their live stuff, ie. Disney Channel.
Does it affect internet speed/connectivity of the PC/Laptop/W.E. while using it?
Speaking for the Chromecast, not at all. But that's probably because I had it about 5 feet away. Just so long as you're not using up all your home's bandwidth, it'll be OK.
Do they freeze up and buffer quite a bit?
Speaking for the Chromecast again, no. It streamed everything just fine. When playing the movies I have on the Plex Server, the movies buffered for a few seconds while loading the movies, but other than that, they all played flawlessly.
Do you need a unit for each tv?
Yes. But if you get a streaming stick, it's a painless process to remove from 1 TV to another. The TV does need to have an HDMI and USB port though. It draws power from the USB port.
For cable cutters, if you already somehow have HBO, Showtime, Hulu Plus subs, it's worth it. If you don't have these subs, you're simply not gonna get much use of this thing unless all you're planning on using it for is YouTube.
I plan on getting the new Amazon Fire Stick soon to replace my old Chromecast. It's faster, newer, comes with a remote and a UI and it's only 25 bucks at Best Buy and Staples. So if you're looking for a streaming stick, you might want to look at this one, unless a Chromecast gets a refresh in the near future.