I've used Bell Trans on two different occasions and was satisfied with their service.
Prices right now:
ON THE STRIP HOTELS: $14.50 round trip or $8.00 one way
OFF THE STRIP HOTELS - $17.50 round trip or $9.50 one way
i believe they have a shuttle every 20mins at the airport. The ride itself isn't too bad, the van's are pretty spacious and the drivers are usually nice. Going to my hotel it took almost 40 minutes though because of all the stops we made.. but I guess it isn't bad if you're trying to be budget friendly since taxi's can cost an arm and a leg in Vegas.
I called in a day before my flight to schedule a pick up at 11:30a, and lucky for us, we were the first stop and passengers on the van (which was more spacious than the first). The van arrived on time, we waited maybe 10mins for other passengers and got to the airport in about 30 mins (with one other stop). Not too shabby!
As with any shuttle, make sure you have enough buffer time from your flights, you just never know!