Bike sizing service - good idea?

It depends.  If those 'consultants' do a true fitting for you and give you the measurements for your contact points, then you are able to shop for those bikes that can best be set up to meet those fit coordinates.  When you go to a shop, you are 'stuck' with whatever bikes they happen to sell (assuming they are not charging for the fit separately and offering it as part of the bike-buying service) and may end up with a bike that offers a 'less good' fit.  But there are plenty of good shops and most will offer some bike that will fit most people reasonably well.  A good fitting is usually worth the money.  Whether you are more comfortable getting that from an outside service or at a specific shop is a more challenging question since it is hard to assess until after you have done it and ridden the bike.

If the service is just to say what size frame you should ride, then it is not worth it.