I'm one of those people who tries Ancestry every time there is a freebie but I have yet to find anything of major interest and nothing I couldn't find elsewhere free -- and better. Why pay for their transcription of a page or a view of a small portion of a page when you can see the actual page in its entirety somewhere else for free? Serious hightly respected researchers have told me they have indeed found good information at Ancestry so perhaps my ancestors did not leave a paper trail that Ancestry has copied. Next time there is a freebie I will check again to see if they have found it yet.
I like Heritage Quest. I love digging through the old public domain books. Unfortunately, the search engine leaves a lot to be desired but if you know the title of the old book you can just search for it. They also have census and Revolutionary War records among other things. Mere mortals cannot get Heritage Quest but their public library can and a great many do have it. If your library doesn't have it ask for it and have your genealogy friends ask for it too. It worked with mine.
Another place I like is Godfrey. It is a bargain at $35 a year. I am particularly fond of their old newspaper collection. I have not discovered anything earth shattering there but I have discovered some interesting news items which add a little excitement to my genealogy. For example, my husband's grandfather was interviewed when he witnessed the crash of a military plane in a residential neighborhood. Who knew? They have other databases too.
Before you pay for an online database see what is available to you free. Start with your local library. Check back because libraries do increase their offerings and drop others that aren't used. Check for a Family History Library near you. Check for your state's online offerings. More and more states are providing records free online -- and some are taking them off unfortunately. Check the offerings at university libraries and archives for all the states where you are researching. The online collections are growing. Goggle for the information. Linkpendium is a great resource.
There are a lot of subscription online databases, from straight commercial to those of the big genealogy societies. Most are not inexpensive. Only you can tell if one is really useful to you. Ask people you know before you buy. See if you can try it out before you buy. See about a short term subscription to see if you really find it useful before you spend the big bucks.