My posts on online trees were fairly negative, so I should mention a few positives that can come of them.
(1) CLUES! Even though a tree may be undocumented, there may be some clues that you can take advantage of: dates - if a birth, marriage or death date is given without a source, make note of it and look for a source for the event.
(2) PARENTS! You may be missing parents for your ancestor. If parents for your ancestor appear on a tree, follow-up. Check census records to see if this couple fits your person. Pre-1850 will be tougher. Look at the ages of the children to see if yours is a fit. 1850 and after, if your ancestor isn't there, don't lose hope. Look at age gaps. If there's a 4-5 year gap where your ancestor would fit, then maybe it's a fit. Look at neighbors to see if your ancestor is living away from home. Search other records to see if this couple could be a fit. Keep searching for confirmation.
(3) SIBLINGS! See procedures under #2 above.
(4) LIFE EVENTS! If the tree mentions military service, another marriage or some other detail, search records to confirm.