Solids save somewhat on development costs and are reasonably low in cost up to the size of the one-piece SRBs used on the Atlas and Delta medium variants. But the Shuttle/Ares SRBs are quite costly to reuse, heavy and hazardous to transport, and require the costly VAB, MLPs and crawlers. The Ariane also uses segmented boosters, although they are nonrecoverable and the fuel grains are cast onsite. However the Shuttle/Ares boosters are more than twice the mass of those on the Ariane. Solids produce more liftoff thrust, so for LEO launches where the payload is relatively heavy but doesn't require as much velocity, they have a relative advantage. For GTO satellite launches and planetary probes, where total acceleration is more important and liftoff mass is less, all-liquid high-ISP launch vehicles like the Delta IV heavy have a relative advantage. I suspect that the heavy versions of the Falcon will come out well ahead of the Ariane V in processing cost for comparable GTO launches, which may be why Ariane needs the Soyuz booster.
However solids are not practical to reuse, as Ariane decided even after adding a recovery system, whereas in theory at least a liquid can simply be refueled and reflown. Practical human spaceflight ultimately can only be achieved with fully reusable systems, so in the long run only fully liquid-propelled launch vehicles will be practical.