If you have ticket for sleeper class (2nd Class), and the rate of shifting of your current waiting list number or RAC number is fast, you have fare chances that your ticket will get confirmed.
Normally for 3rd AC, 2nd AC and 1st AC and on Rajdhani express, the chances of RAC / Wait listed Ticket getting confirmed is less. Also, if the date of journey is near to any festival season, chances of confirmation of ticket is less in that situation. Even if you have RAC 2 or RAC 3 status before 2-3 days of journey, their may still be chances that ticket will not get confirmed after chart preparation. Anything can happen in Indian railway.
If you book the ticket with RAC, long before the journey, you must be insured that your ticket will get confirmed.
Problem comes for those people who book the ticket in wait listed number and the day of journey is very near to booking, they must prepare for non-confirmation of ticket, or they may go with RAC ticket.
Traveling with RAC ticket is worst in Indian Railway, their is no online facility to know whether your RAC ticket got confirmed or not while traveling. All the information is kept with TTE. Only TTE knows the status of your RAC ticket, you have to ask frequently from TTE about whether or not you got the confirmed seat for RAC ticket.
The other problem with RAC is that two people share the same birth while traveling, even if you paid full price for the birth, you may not sleep on the birth.
To avoid worst travel with RAC, you should try to take Tatkal ticket, when you book Tatkal ticket, you will either get confirmed ticket or waitlisted ticket, that waitlisted ticket may convert to confirmed ticket only or your full money will get refunded.