Anyone that knows how I shoot knows I rate the speedlites (heck, ive even been in situations with top of the line Profoto gear to hand, and still I take out my wee SB26s!).
As Scott points out, light is light. Regardless of whether it is produced by a studio strobe, a speedlite or even the sun itself, its just light (if you don’t believe me check Scotts comparison examples and see if you can spot the difference!) Its what you do with it that makes the difference.
This is the main reason I will sometimes choose studio strobes over the little SBs- there simply is just more available to shape the light its pushing out. That said, with the rise in popularity of the strobists there is a great deal available nowadays to put on the end of your speedlite and take control of the light.
The other main advantage studio strobes have over speedlites is their modelling light. This really helps you see where the shadows fall on your subject, but its not essential. You just need to learn to predict how the light will fall, fire a few test shots to make sure you are in the ballpark, and just fine tune.
Sure studio strobes are a lot more powerfull, but with todays advances in acceptable ISO levels, to me at least its not such a big deal, unless you are attemting to overpower the sun at midday. That’s a job for the big guns!