Any discussion of flight planning services is a sure fire way to get me (and most pilots who use them) all fired up. It's rare that you find one that is consistently prompt and accurate. Most are better in one region or another.
In my experience, most are flat out incompetent or just lazy. For a time, we were forced to use one that routinely would file for an overflight permit on the leg out, but not the return. At most airports, the ground handeler works for all the planning companies (and is usually the brother in law of the airport manager).
Be careful if you are doing a charter, it can take six months or a year to get all your bills! Make sure to find out all the estimated charges and bill them up front.
These days, I'd vote for Air Routing (They were just sold to Rockwell, so we'll see how long it takes for them to go downhill) or AirInc.
FR