Calling it open source is a bit dirty. The data slurping part is, but their stuff on the other end is completely proprietary and paid.
The software running on your machines is open source. It can be reviewed. Quite important for people not wishing to run closed source software on their machines.
Yea, if you're hosting with them, trusting them is handy.
This Longview agent can be run anywhere though, doesn't have to be on a Linode machine. Not entirely sure why you'd want this setup, but still..