I used this last year and it was straightforward enough, got picked up outside Seaworld and it picked us up outside the entrance point of BG early evening. If you're staying on International Drive it picks up at various points along there and I'm sure it's free if you have the 5 park flexiticket. I wouldn't bother renting a car just to go to BG, if you're not getting one anyway then the shuttle bus is fine.