We drove from SC last February. I can't stand I-4 with congestion. As others state, if you are not hitting it at rush hour, it still moves, but there is still traffic. Did some research, and 417 now connects directly to I-4 in Sanford. Very clearly marked. AAA did not have this exit on their maps yet. This is a toll road, and does cost about $6.00. Very little traffic, and a very relaxing ride. WDW exits are 6 and 3. Since going to Coronado Springs, we took exit 3. (warning, exit 3 has a $0.25 toll, which requires exact change).
I only will go 417 now. (Just did it again in June.) To me, the toll money is worth the more relaxing ride. I don't like getting to WDW all tensed up from the ride. The length is definitely a few miles longer, but you are travelling at 65 or 70 mph.
To make it easier for future trips, get information on the EZ pass. It might be worth getting a transponder, so you don't have to slow down to pay toll.