Depends on scale and on budget. At very high scales (data-center that needs peta-byte level storage) solid state technologies are just not sufficiently cost competitive yet. For a PC where the scale is say 200G->2T or so they are on the cusp. Prices are around the $0.60/GB range currently, so the 200G->2T works out as around $120->$1200 (actually more at the lower end, because larger SSDs are cheaper per gigabyte). HDDs are still something like an order of magnitude cheaper, but as an overall fraction of the cost of a system the additional cost is much less as an overall percentage.
My advice would be small SSD for system drive + HDD for data if you want to keep costs down, but if you can afford to go all SSD it's definitely a benefit for many applications (not for things like video or photo storage, but for things that need fast random access - loading C2C, concurrent multiple VMs, ...).