I fly to and from Tampa all the time... and drive to and from WDW from Tampa all the time. If I was just going to WDW, though, for $50 I'd fly to Orlando. If you have 5 or 6 people and you're going to save $250-$300 total, though, then consider Tampa.
As others have pointed out, the traffic can be awful. I-4 at Himes is actually considered the central point in America's Worst Commute, according to Fortune or some other magazine earlier this year. But you can hit bad traffic anywhere along the route. Doesn't mean you will -- but you certainly could.
Without traffic, I can make it from Tampa airport to WDW in about 70 minutes without breaking too many laws. If you've got a leadfoot (and no traffic), you could do it quicker.