IMHO, it provides the ultimate in setup flexibility and on-vehicle packaging, but unless you can identify a clear shortcoming in your separate coil and shock arrangement compared to your needs, and can set up your coilovers properly, it's not worth the cost, fab and tuning.
Running a factory radius arm front?
Running factory steering balljoints?
Wagon and don't want to cut holes in your floor?
Would I switch to coilovers? Yes
Does what I do with my truck at the moment justify it? No.
There's plenty that can be done with near factory setups, modified shock mounts and different coils etc, but most people don't do much more than bolt in an off the shelf lift kit.