I have used lawn services several times over the years, always with success. I think the biggest potential issue is that they may apply the fertilizer [[or whatever it is) during a dry spell, in which case you should water your lawn within 24 hours [[if memory serves me), or if you happen to get a lot of rain right after an application it may wash the stuff right off before it has a chance to do it's work. But like I said, I've never had noticable issues. I compared the cost of having a service versus doing it myself, and doing it myself would actually probably be slightly more expensive [[and obviously more time consuming).