We use a few other services for this and we compared them on a dozen numbers in Europe & US. Unfortunately, the answers were mostly wrong or inaccurate. They mostly the first network owner of the number, not the current one. Plus they give you the original legal name of the company, not the current one (like Cingular instead of AT&T).
It might be useful if you just need to know if the number 'exists' but not if you need to know the actual carrier.