From the Sefer Yetzirah it seems that there is both a hard and soft letter reish (see 4th perek, Aryeh Kaplan edition). How do you pronounce the soft reish?
In the Hebrew dialect of Habbani Jews (type of Temanim), we maintain a double pronunciation of resh, as well as a few other sounds I'm not aware of other dialects having.
The main pronunciation of resh is a regular rolled r, like exists in Arabic and Spanish. The soft resh is much like an English r, but more emphatic, as if you were about to roll it but stopped short. You can still hear this sound in some old Arabic music. The fact that this sound existed in earlier Arabic I'm sure is along the lines of more than 50% of root words in Arabic come from Hebrew - not the other way around.
Interestingly, in Habbanit our hard gimel is neither j nor g, but a kind of clicking g sound. (Soft gimel is "gh") Samekh is best described as the s sound after the k in "Mexico" (since x is truly k-s), with the tongue positioned lower than in a regular s sound. Quf is a guttural "g" sound, not as intense as the Iraqi version. Everything else is the same as normative Temani dialect.
This stuff fascinates me. ^_^
Hidāyat al-Qāriʾ (see Eldar's edition) indicates that the regular resh pronunciation was articulated at the middle of the tongue. This is farther out than the articulation for fricative gimel (IPA: ʁ) or the fricative kaf (IPA: χ), which was articulated "at the back third of the tongue". This suggests that resh had an advanced uvular articulation. It is unclear if it is a roll (IPA: ʀ) or frictionless continuant (IPA: ʁ̖).
The second resh (called "heavy"), which is found in the environment of the alveolar consonants דזצתטסלן was probably an alveolar trill, like the one used for resh in Middle Eastern Hebrew. The terminology used to refer to this resh (makrūḵ) and other evidence suggests that this resh was emphatic.
Saʿadya Gaʿon writes in his commentary on Sefer Yeṣirah that Tiberians have a dual form of resh (hard and light), but the Babylonians do not.
See Khan's A Short Introduction to the Tiberian Masoretic Bible and its Reading Tradition.