At my parish, nearly all the priest's parts are done in Serbian (with perhaps a Small Litany in Slavonic, and a mixed Serbian/English litany here and there), while the standard responses are sung in Church Slavonic. (I have only very seldom heard any standard responses sung in the Serbian language anywhere.) Troparia and kontakia, together with the stikhira at the other Services, more often than not are done in Serbian--though things like the troparion/kontakion of the Temple, O Protection of Christians, and the like are usually sung in Church Slavonic. The Epistle is read in either Serbian or English (depending on the reader), and the Gospel always in Serbian and English.
A nearby parish, however, uses Church Slavonic exclusively; this was formerly the local Free Serbian parish. As a matter of fact, I find that more often than not the former Free Serbian parishes tend to have services in Church Slavonic, whereas Serbian is rather widespread elsewhere. Bishop Longin himself, as I have heard them express more than once, prefers to serve in Church Slavonic and does so exclusively at Nova Gračanica.