From a synthetic point of view, the Sn1 reaction is less useful. It is prone to side reactions such as eliminations and carbocation rearrangements.
The Sn1 mechanism:
a) is a multistep process
b) occurs with formation of carbocation intermediates in the rate determining step
c) involves one transition state per step. The rate-determining step involves a high polarity transition state
d) follows first order (unimolecular) kinetics. That is, rate=k[substrate]
In nucleophilic substitutions at sp3 carbon, Sn1 mechanisms are favored by using:
a) sterically hindered substrates
b) weak (neutral), small nucleophiles
c) moderate to high polarity solvents that can stabilize the transition state and the carbocation intermediate
Stereochemically, if the electrophilic center in the substrate is chiral, the Sn1 reaction produces a racemic product. The relative proportions of the enantiomers depend on the specific reaction, but will typically be close to 50/50.