I used a diaper service for Katelyn's first couple months and loved it! I think it's ideal for a new baby because you get a size that fits just perfectly, can switch sizes so easily, and someone else comes to your door once a week to take away your dirties and bring you cleans! Anything to make life easier those first few months!
I beleive I paid around $13 a week for our's, then on top of that I rented the covers from them which was a little more. I would say about the same as buying sposies.
A few things I didn't like about it (and why I am no longer using one):
1. No choices -- all prefolds and white covers. Of course, you could buy your own covers, but you always have prefolds. No cute AIO's and fitteds.
2. I think the quality of the prefolds from a diaper service are a little less than some of the great ones made of hemp or natural unbleached cotton are. For a newborn it worked great, but as Katelyn got older they just were not absorbant enough for her. The diaper service recommended using two prefolds, but that gets bulky.
I'd check out your local ones and see what you think. Ask them how they wash the diapers, if they use a bacteriostat to help with stink and rashes, if they offer AIO's or it they will wash some of your own stock...things like that. Beware, they DO use bleach, so if you throw in a colored diaper or washcloth, it's coming back white!!