I have seldom had an issue with any Disney transportation. When using DME to get to the resort, it takes an average of 70 mins..from gate to resort. Your bags do take a bit longer....up to 4 hrs after resort checkin (not arrival at the airport!!).
Then, it looks like you'll be at BLT..so let's figure on that resort....you are at BLT and you want to go to, say, AK. Ok, you go down to the bus stop and wait for the AK bus to come. You could have a wait of 2 mins or you could have just missed the bus and face a 15-20 min wait. Versus, having a rental car. You walk out to the parking lot, get into your car and then drive to AK, where you now need to find a parking spot and either walk to the park entrance, or take the tram. Then, reverse that when it's time to leave.
Of course, when using Disney buses, you will be sharing a route with a few other resorts (or, multiple stops within your own resort). I can't remember what resorts CR/BLT now share with..but it's probably two other resorts.
So...if you are the type person who just wants to 'get going' and stop dillydallying, then a rental car may be the best bet. I don't mind the extra time spent using the buses, but others hate it.