So, as others have mentioned electric imp is in the business of making the best secure platform for people to build IoT devices on top of.

If you can determine that there's an electric imp in the IoT device you're looking at, I personally guarantee that that device is - at least as far as DDoS participation and interfering with your traffic or home network - totally trustable.

Whether the device actually does its job well is a totally different problem, but building a trustable, secure platform is absolutely what we've been spending years doing.

As for other devices: I personally would have a lot of trouble trusting most devices built on embedded linux these days. They can be secured, but pretty much nobody actually does secure them. This is rather a sad state of affairs

As for protecting yourself against IoT devices making your home network insecure: don't have any open ports on your router and turn uPnP off. Do not poke holes in your NAT/inbound firewall for arbitrary devices, because it turns out most of them will be exploitable.