I use both workstations and VSTs, and I also use hard synths. For my needs nothing can replace a good keyboard with good built in sounds that I can use at a moments notice without having to boot up a PC and load VSTs. I'll also echo zircons pros and cons, with the exception of integration into my everyday music needs. I find that workstation sounds sit extremely well in any song, as they tend to have a rounder and more "complete" sound than VSTs (though they do lack the realism factor that you get from multisampling and all that jazz that VSTs do). But in the end it's really 80% performance and 20% samples, and when I hear a blatant rompler sound in a song I feel warm and fuzzy cuz it's just a cool sound that's playing a cool passage, but when I hear a blatant VST sound trying desperately to sound real (and 99% of the time VSTs don't sound real) I cringe and can't listen to that song anymore.
The only exception to this rule are string and brass sounds. Orchestral VSTs always sound great to me, not real, but great. It's like hearing a really good Neo Geo guitar patch, you know it's fake but it sounds great regardless.