Taking a flight back home alone, I opted for an airport shuttle for $11 as opposed to taking a taxi by myself for ~$20. I booked a reservation for a shared van the night before with ease. Day of, I received an automated call 15 minutes earlier than scheduled informing me that my driver was downstairs.
I stayed at Mandalay Bay. After lingering around the taxi area for a while, I asked and found out that the shuttles wait downstairs on the beach level where the valet pick-up area is. The shuttle was indeed there and waiting.
I ended up being the only one on the van and arrived at the airport earlier than expected. I had added gratuity when paying for my reservation and my driver appeared aware, though I did not ask much about it. I sensed no awkwardness luckily -- I was afraid the driver would still expect a tip at the end. (There's signs on the vans reminding you that gratuity is not included with the fare.)
Overall, I'd definitely recommend taking SuperShuttle if you need to get to the airport alone. Fast, safe, and convenient!