I used bookit for my wedding. About 2/3 of the guests booked through Bookit. It was easy and quite a bit cheaper.
A bit of advice/tips:
1) Make sure you have selected the flights that are best for you - allow enough time for layovers if not flying direct. Different flight options cost different amounts.
2) If booking children under 2 - do not include them in your reservation as it will not process. After booking call the airline directly to add a child under 2 to your reservation. This may cost extra (for my nephew it was approximately $150 with Air Canada)
3) The reps at the call center are hit-or-miss. Some of their reps are amazing, some not so much.
4) You don't have to pay all at once!!! You can split your balance into 3rds. The airfare is due upfront, but the resort cost can be paid off in intervals.
5) Round trip transfers are not included with bookit, but I think they are with Apple. My family used USA transfers at the Cancun airport and they were great. They booked ahead and saved a lot (on bookit it's 45$ per person - that's a lot!
Any questions, just ask!