Using other trees can be useful to give a guide, provided the other person's work is solid - that the biggest problem I have with MyHeritage vs Ancestry.
I've found that researchers on MyHeritage seem to be a lot more lax in their proof standards, and make the mistake of replicating other people's mistakes with just automatically approving their Smart Matches without checking.
I'm using RootsMagic 7 for my software, and the paid version does allow you to log into familysearch.org, MyHeritage, and Find My Past, and it's working on collaborating with Ancestry to take over from FTM, so that might be something you want to look into. They regularly have sales, so it can be really affordable.
It's also got a hint system that's very similar to Ancestry's, but covers those other sites as well, so it'll show you info on familysearch.org or MyHeritage if you've logged into those sites.
Find My Past, I've found, is more helpful in researching actual documents vs. other's trees, and it has a similar hint system to Ancestry. I've been using it recently, and I rather like it. If you're interested, keep an eye out for one of their % off specials and then give it a 14 day free trial.
familysearch.org is the Mormon's genealogy site - the goal there is for there to be only one entry per identity, so it's a massive communal family tree, and it's 100% free. They do follow a solid standard of research and have an alert system for data errors/missing records, and I've found that to be immensely useful in locating difficult people.
Having said all of that, as the others mentioned, it's absolutely imperative that you source your own records, once you have a potential family member located.
Always check, double check, and back research anyone you find in an online tree, and make sure there's no conflicts between the info you have and the info you're being offered.
A great example from my own tree - an earlier researcher had attached a Margaret Mary Courtney who was registered in a UK census, aged 5, as my grandmother's mother, and everyone just accepted that, and kept replicating that data, tree after tree after tree, and I made the same mistake.
As great-grandma died giving birth to my grandmother, there was little family info to work with, and it was all in the right time period, the records were there, so we weren't questioning it.
Then I located Margaret's death record in the Tasmanian archives, instead of just a transcript, and discovered that she was listed as 'native born' of Tasmania, to a convict mother.
She simply could not have been 5 years old in England having been born to a convict in Australia at the same time.
Having found that, I was able to locate Margaret's parents, sister, and all of their marriage, birth, death, and burial records. Had I done my own research into Margaret earlier, I wouldn't have made that mistake. Instead, I'd spent a lot of time unnecessarily, trying to determine when Margaret emigrated.
Personally, I also like to source any newspaper notices that might be around to confirm my research, as this can be an enormous source of incidental details that can prove or disprove the person's family connections, too.
