I've used Supershuttle from John Wayne to DLR hotels a number of times. I've had no complaints. Supershuttle is probably the best known shuttle in LA, but there are others that are also as good. The fare was around $11 each way + tip. If there are 4 or more in your party, it's cheaper to take a taxi. A cab should run you about $36 plus tip, and there won't be other stops.
For the shuttles, you will need reservations from DL to John Wayne, but generally not visa versa. It shouldn't hurt to make reservations since you should be under no obligation and shouldn't have to pre-pay. Usually, they don't charge your credit card until you catch the ride. Even if you make reservations, you might find another company, depending upon the timing.
I've also taken the bus. It's easy to catch, but unless you time it just right and depending upon where you are staying, it can take quite a bit longer. I don't think its worth the slightly cheaper fare.