What is the proper etiquette and technique to using other peoples research, specifically Ancestry.com trees or info posted on websites?

When I started researching I spent an entire afternoon copying Ancestry.com trees. 900 years worth of inaccurate lineage. I simply did not know any better. I assumed that whoever had gone through the trouble of uploading all that info must've done their homework. I had to spend the next -week- deleting everything. It was a tough way to learn a lesson but at least I realized early on the error of my ways before spending months or even years chasing the wrong family.

Now I am pre-researched family phobic. In fact, I've pretty much ignored my entire paternal line because much of the research is 'done' on those lines. I raise my eyebrow and think "Ohhh, reallllly...." every time I pull up a tree tracing my ancestors back to the stone age (or the 1600's). It's like when someone gets cheated on and is afterwards suspicious of anyone they date.

The problem is, I fear I may be missing out of some great research by being so skeptical. For instance, today I came across a tree of someone who had been researching the same Douglas line from Northumberland Co., VA. Not only had they attached their Ancestry.com sources, they added details about sources like tombstones and marriage registers that had obviously been obtained offline.

It would be extremely impractical for me to go back and retrace what appears to be years of research. I live hours away from the area and do not have the money or time to obtain each record they cite. What I guess I'm asking is - when can we feel comfortable taking at face value this type of information? Can we ever?

A follow up question would be, what is the acceptable way to take and use this info? I don't want to come in and copy all of someone's hard work. Yet, at the same time, I'm not going to research from scratch information that has appears to have already been well validated.

I'd love to hear any opinions.