I will give you 3 things to look for to ensure you get the best information possible.
1) Fast access to information: The better of these places will be able to give you the information that you are after almost instantly.
Your results basically should be available within seconds of pressing a search button.
Compare this to trying to get this from Google.
If any relevant results are returned, you will have to manually look through the pages to try and find it.
This is basically a huge waste of time.
2) Accurate Information: This is an area that there can be a great difference between one site and the next.
There is a very straightforward reason that this is the case.
The cell service providers do not make their customer's data readily available to anyone.
So, if you ever need to research a cell number, you will have to use a site that has access to them.
Only the best of the sites have taken the effort necessary to gather this information and keep it constantly up to date.
3) Easy to use: This is something that may have never really occurred to you.
But, let me give you an example of what I am talking about.
I have personally seen sites with up to one dozen boxes to enter a number depending on what kind of information that you are after.
This is a completely unnecessary level of complexity.
The best of these sites will have only one box to enter a number into and they will be able to figure it out for themselves.